emacs: Major readme update and refactor

Also update the license and switch to GPL v2 like lammps

tools/emacs/README.md

+# GNU Emacs Syntax Highlighting
+
+> Copyright (C) 2010-2018  Aidan Thompson <athomps at sandia.gov>
+> Copyright (C) 2018  Rohit Goswami <r95g10 at gmail.com>
+
+The `lammps-mode.el` file provided in this directory will enable syntax
+highlighting for the lammps script syntax in GNU Emacs. The groupings of
+commands were originally copied from `tools/vim`.
+
+## Installation
+**Requirements: GNU Emacs 24.\***
+
+### Obtaining the Package
+
+#### MELPA
+
+The easiest installation method is via MELPA and it is advisable to use one of
+the many [MELPA installation methods](https://melpa.org/#/getting-started).
+
+For example, with [use-package](https://github.com/jwiegley/use-package) one can
+simply use the following:
+
+``` emacs-lisp
+(use-package lammps-mode)
+```
+
+#### Manually
+
+Assuming for some reason you have downloaded the file to `~/.emacs.d/lisp` you
+would do the following (kanged [from here](http://ergoemacs.org/emacs/emacs_installing_packages.html)):
+
+``` emacs-lisp
+;; Tell emacs where is your personal elisp lib dir
+(add-to-list 'load-path "~/.emacs.d/lisp/")
+
+;; load the package.
+(load "lammps-mode")
+```
+
+### Autoloading \& Auto-recognition
+
+For autoloading and auto-recognizing `in.*` and `*.lmp` files add the following
+to `.emacs`:
+
+``` emacs-lisp
+(autoload 'lammps-mode "lammps-mode.el" "LAMMPS mode." t)
+(setq auto-mode-alist (append auto-mode-alist
+                              '(("in\\." . lammps-mode))
+                              '(("\\.lmp\\'" . lammps-mode))
+                              ))
+```
+
+## Status
+
+By far not all commands are included in the syntax file (lammps-mode.el). You
+can easily add new ones to the existing classes.
+
+## Implementation Details
+
+`lammps-mode` is derived from `shell-script-mode` which provides some basic
+syntax highlighting of strings, comments, etc.
+
+The MELPA recipe used for this package is simply:
+
+``` emacs-lisp
+(lammps-mode :fetcher github :repo "HaoZeke/lammps-mode")
+```
+ 
+## Caveats
+
+* Does not work with Xemacs [See [this comment](https://github.com/lammps/lammps/pull/1022#issuecomment-408871233)]
+
+## License
+
+[GNU GPL v2](https://github.com/HaoZeke/lammps-mode/blob/master/LICENSE).
+Check the file for more details.

tools/emacs/README.txt

-=== Emacs Syntax Highlighting ===
-Created by Aidan Thompson 12/2010
-===============================
-Updated by Roit Goswami Mon Jul 30 2018 
-
-The lammps.el file provided in this directory will enable syntax 
-highlighting for the lammps script syntax in emacs. The groupings
-of commands were copied from tools/vim. The simulation scripts have to
-end on *.lmp or start with in.* (see lammps.el). By far not all 
-commands are included in the syntax file (lammps.el). 
-You can easily add new ones to the existing classes.
-'lammps-mode' is derived from 'shell-script-mode' which provides
-some basic syntax highlighting of strings, comments, etc.
-
-=To enable the highlighting:
-============================
-(0) Create/edit the emacs init file ~/.emacs to contain:
-   
-(load "~/.emacs.d/lammps-mode.el")
-
-This file may also be called ~/.emacs.el, or ~/.emacs.d/init.el
-
-(1) Copy lammps-mode.el to the directory ~/.emacs.d
-
-=Update:
-========
-
-The package may now also be installed by a MELPA style recipe, namely:
-
-```lisp
-(lammps-mode :fetcher github :repo "HaoZeke/lammps-mode")
-```
-
-For a simpler installation with `use-package` simply add:
-
-```lisp
-(use-package lammps-mode)
-```
-
-The latest version of the package will be kept in sync as a squashed update on
-the lammps repository as well.
-
-It is advisable to use the MELPA installation methods listed here:
-https://melpa.org/#/getting-started
-
-For autoloading and auto-recognizing "in.*" and "*.lmp" files add the following
-to `.emacs`:
-
-```lisp
-(autoload 'lammps-mode "lammps-mode.el" "LAMMPS mode." t)
-(setq auto-mode-alist (append auto-mode-alist
-                              '(("in\\." . lammps-mode))
-                              '(("\\.lmp\\'" . lammps-mode))
-                              ))
-```

tools/emacs/lammps-mode.el

@@ -14,20 +14,19 @@
 
 ;; This file is not part of GNU Emacs.
 
-;; This file is free software; you can redistribute it and/or modify
+;; This program is free software; you can redistribute it and/or modify
 ;; it under the terms of the GNU General Public License as published by
-;; the Free Software Foundation; either version 3, or (at your option)
-;; any later version.
+;; the Free Software Foundation; either version 2 of the License, or
+;; (at your option) any later version.
 
-;; This file is distributed in the hope that it will be useful,
+;; This program is distributed in the hope that it will be useful,
 ;; but WITHOUT ANY WARRANTY; without even the implied warranty of
 ;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 ;; GNU General Public License for more details.
 
-;; You should have received a copy of the GNU General Public License
-;; along with GNU Emacs; see the file COPYING.  If not, write to
-;; the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
-;; Boston, MA 02111-1307, USA.
+;; You should have received a copy of the GNU General Public License along
+;; with this program; if not, write to the Free Software Foundation, Inc.,
+;; 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 
 ;;; Commentary:
 ;; translation of keyword classes from tools/vim