Add CONTRIBUTING file with a standard contributor Git how-to (#237)

Described command line way, fetches only upstream (to not do things not needed),
focuses on code, mentions issues, leaves out compilation.

CONTRIBUTING.md

@@ -0,0 +1,174 @@
+# Contributing
+
+## Contributions other than code
+
+There is more than one way of contributing, see full list at
+https://grass.osgeo.org/get-involved/.
+In the rest of this document, we will focus on contributions centered
+around the GRASS GIS source code.
+
+## Reporting issues and suggesting features and changes
+
+* Get an OSGeo Services UserID (http://www.osgeo.org/osgeo_userid).
+* Got to https://trac.osgeo.org/grass/#BugTracking and follow instructions there.
+
+## Changing code and documentation
+
+This guide covers contributing to the main version of GRASS GIS source
+code which is the master branch.
+It assumes that you have some very basic knowledge of Git and GitHub,
+but if you don't just go through some tutorial online or ask on the
+GRASS GIS developer mailing list.
+
+### First time setup
+
+* Create an account on GitHub.
+* Install Git on your computer.
+* Set up Git with your name and email.
+* Fork the repository.
+* Clone your fork (use SSH or HTTPS URL):
+
+```
+git clone git@github.com:your_GH_account/grass.git
+```
+
+* Enter the directory
+
+```
+cd grass/
+```
+
+* Add main GRASS GIS repository as "upstream" (use HTTPS URL):
+
+```
+git remote add upstream https://github.com/OSGeo/grass
+```
+
+* Your remotes now should be "origin" which is your fork and "upstream" which is this main GRASS GIS repository. You can confirm that using:
+
+```
+git remote -v
+```
+
+* You should see something like:
+
+```
+origin	git@github.com:your_GH_account/grass.git (fetch)
+origin	git@github.com:your_GH_account/grass.git (push)
+upstream	https://github.com/OSGeo/grass.git (fetch)
+upstream	https://github.com/OSGeo/grass.git (push)
+```
+
+It is important that "origin" points to your fork.
+
+### Update before creating a branch
+
+* Make sure your are using master branch:
+
+```
+git checkout master
+```
+
+* Download updates from all branches from all remotes:
+
+```
+git fetch upstream
+```
+
+* Update your local master branch to match master in the main repository:
+
+```
+git rebase upstream/master
+```
+
+### Update if you have local branches
+
+If `rebase` fails with "error: cannot rebase: You have unstaged changes...", then move your uncommitted local changes to "stash" using:
+
+```
+git stash
+```
+
+* Now you can rebase:
+
+```
+git rebase upstream/master
+```
+
+* Apply your local changes on top:
+
+```
+git stash apply
+```
+
+* Remove the stash record (optional):
+
+```
+git stash pop
+```
+
+### Creating a branch
+
+Now you have updated your local master branch, you can create a branch
+based on it.
+
+* Create a branch and switch to it:
+
+```
+git checkout -b new-feature
+```
+
+### Making changes
+
+You can use your favorite tools to change source code or other files
+in the local copy of the code. When make changes, please follow
+Submitting Guidelines at
+http://trac.osgeo.org/grass/wiki/Submitting.
+
+### Committing
+
+* Add files to the commit (changed ones or new ones):
+
+```
+git add file1
+git add file2
+```
+
+* Commit the change:
+
+```
+git commit -m "Added a new feature"
+```
+
+### Pushing changes to GitHub
+
+* Push your local branch to your fork:
+
+```
+git push origin new-feature
+```
+
+### Pull request
+
+When you push, GitHub will respond back in the command line to tell
+you what URL to use to create a pull request. You can follow that URL
+or you can go any time later to your fork on GitHub, display the
+branch `new-feature`, and GitHub will show you button to create
+a pull request.
+
+### After creating a pull request
+
+GRASS GIS maintainers will now review your pull request.
+If needed, the maintainers will work with you to improve your changes.
+
+Once the changes in the pull request are ready to be accepted,
+the maintainers will decide if it is more appropriate to:
+
+* merge your branch,
+* squash all commit into one commit, or
+* rebase (i.e., replay) all commits on top of the master branch.
+
+### Further notes
+
+GRASS GIS maintainers use additional workflows besides the one described
+above. These are detailed at https://trac.osgeo.org/grass/wiki/HowToGit

SUBMITTING

@@ -1 +1 @@
-See http://trac.osgeo.org/grass/wiki/Submitting
+See CONTRIBUTING.md and http://trac.osgeo.org/grass/wiki/Submitting