make it more explicit, that master needs to be updated and feature branches...

make it more explicit, that master needs to be updated and feature branches should be created from master

doc/src/tutorial_github.txt

@@ -85,18 +85,20 @@ test them without interfering with the repository on Github.
 To pull changes from upstream into this copy, you can go to the directory
 and use git pull:
 
-  $ cd mylammps :pre
+  $ cd mylammps
+  $ git checkout master
   $ git pull https://github.com/lammps/lammps :pre
 
 You can also add this url as a remote:
 
   $ git remote add lammps_upstream https://www.github.com/lammps/lammps :pre
 
-At this point, you typically make a feature branch for the feature you
-want to work on. This tutorial contains the workflow that updated this
-tutorial, and hence we will call the branch "github-tutorial-update":
+At this point, you typically make a feature branch from the updated master
+branch for the feature you want to work on. This tutorial contains the
+workflow that updated this tutorial, and hence we will call the branch
+"github-tutorial-update":
 
-  $ git checkout -b github-tutorial-update :pre
+  $ git checkout -b github-tutorial-update master :pre
 
 Now that we have changed branches, we can make our changes to our local
 repository. Just remember that if you want to start working on another,
@@ -111,15 +113,14 @@ After everything is done, add the files to the branch and commit them:
 
 [Warning]
 
-{"Do not use "git commit -a".  the -a flag will automatically include
-*all* modified or new files.  mercurial does that and it find it
-hugely annoying and often leading to accidental commits of files you
-don't want.  use git add, git rm, git mv for adding, removing,
-renaming and then git commit to finalize the commit.  personally, i
-find it very convenient to use the bundled gui for commits, i.e. git
-gui.  typically, i will do git add and other operations, but then
-verify and review them with git gui.  git gui also allows to do
-line-by-line unstaging and other convenient operations." - Axel}
+{Do not use `git commit -a`.  The -a flag will automatically include
+_all_ modified or new files and that is rarely the behavior you want.
+It can easily create to accidentally adding unrelated and unwanted
+changes into the repository. It is highly preferable to explicitly
+use `git add`, `git rm`, `git mv` for adding, removing, renaming files,
+respectively, and then `git commit` to finalize the commit.  If you
+find doing this on the command line too tedious, consider using a GUI,
+the one included in git distributions written in Tk, i.e. use `git gui`.}
 
 After adding all files, the change can be commited with some useful message
 that explains the change.