create missing "Build_manual.txt" file from 'doc/README'

doc/src/Build_manual.txt

+"Previous Section"_Errors.html - "LAMMPS WWW Site"_lws -
+"LAMMPS Documentation"_ld - "LAMMPS Commands"_lc - "Next
+Section"_Manual.html :c
+
+:link(lws,http://lammps.sandia.gov)
+:link(ld,Manual.html)
+:link(lc,Commands_all.html)
+
+:line
+
+Building the LAMMPS Manual :h2
+
+Depending on how you obtained LAMMPS, the doc directory has 
+2 or 3 sub-directories and optionally 2 PDF files and an ePUB file:
+
+src             content files for LAMMPS documentation
+html            HTML version of the LAMMPS manual (see html/Manual.html)
+tools           tools and settings for building the documentation
+Manual.pdf      large PDF version of entire manual
+Developer.pdf   small PDF with info about how LAMMPS is structured
+LAMMPS.epub     Manual in ePUB format :pre
+
+If you downloaded LAMMPS as a tarball from the web site, all these
+directories and files should be included.
+
+If you downloaded LAMMPS from the public SVN or Git repositories, then
+the HTML and PDF files are not included.  Instead you need to create
+them, in one of three ways:
+
+(a) You can "fetch" the current HTML and PDF files from the LAMMPS web
+site.  Just type "make fetch".  This should create a html_www dir and
+Manual_www.pdf/Developer_www.pdf files.  Note that if new LAMMPS
+features have been added more recently than the date of your version,
+the fetched documentation will include those changes (but your source
+code will not, unless you update your local repository).
+
+(b) You can build the HTML and PDF files yourself, by typing "make
+html" followed by "make pdf".  Note that the PDF make requires the
+HTML files already exist.  This requires various tools including
+Sphinx, which the build process will attempt to download and install
+on your system, if not already available.  See more details below.
+
+(c) You can genererate an older, simpler, less-fancy style of HTML
+documentation by typing "make old".  This will create an "old"
+directory.  This can be useful if (b) does not work on your box for
+some reason, or you want to quickly view the HTML version of a doc
+page you have created or edited yourself within the src directory.
+E.g. if you are planning to submit a new feature to LAMMPS.
+
+:line
+
+The generation of all documentation is managed by the Makefile in
+the doc dir.
+
+Documentation Build Options: :pre
+
+make html         # generate HTML in html dir using Sphinx
+make pdf          # generate 2 PDF files (Manual.pdf,Developer.pdf)
+                  #   in doc dir via htmldoc and pdflatex
+make old          # generate old-style HTML pages in old dir via txt2html
+make fetch        # fetch HTML doc pages and 2 PDF files from web site
+                  #   as a tarball and unpack into html dir and 2 PDFs
+make epub         # generate LAMMPS.epub in ePUB format using Sphinx 
+make clean        # remove intermediate RST files created by HTML build
+make clean-all    # remove entire build folder and any cached data :pre
+
+:line
+
+Installing prerequisites for HTML build :h3
+
+To run the HTML documention build toolchain, Python 3 and virtualenv
+have to be installed.  Here are instructions for common setups:
+
+Ubuntu :h4
+
+sudo apt-get install python-virtualenv :pre
+
+Fedora (up to version 21) and Red Hat Enterprise Linux or CentOS (up to version 7.x) :h4
+
+sudo yum install python3-virtualenv :pre
+
+Fedora (since version 22) :h4
+
+sudo dnf install python3-virtualenv pre
+
+MacOS X :h4
+
+Python 3 :h5
+
+Download the latest Python 3 MacOS X package from
+"https://www.python.org"_https://www.python.org
+and install it.  This will install both Python 3
+and pip3.
+
+virtualenv :h5
+
+Once Python 3 is installed, open a Terminal and type
+
+pip3 install virtualenv :pre
+
+This will install virtualenv from the Python Package Index.
+
+:line
+
+Installing prerequisites for PDF build
+
+[TBA]
+
+:line
+
+Installing prerequisites for epub build :h3
+
+ePUB :h4
+
+Same as for HTML. This uses the same tools and configuration
+files as the HTML tree.
+
+For converting the generated ePUB file to a mobi format file
+(for e-book readers like Kindle, that cannot read ePUB), you
+also need to have the 'ebook-convert' tool from the "calibre"
+software installed. "http://calibre-ebook.com/"_http://calibre-ebook.com/
+You first create the ePUB file with 'make epub' and then do:
+
+ebook-convert LAMMPS.epub LAMMPS.mobi :pre
+