Merge pull request #4931 from jsquyres/pr/contributing-guideliens

CONTRIBUTING.md: add Github contribution guidelines

CONTRIBUTING.md

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+# Contributing to Open MPI
+
+First off, thank you for taking the time to prepare a contribution to
+Open MPI!
+
+![You're awesome!](https://www.open-mpi.org/images/youre-awesome.jpg)
+
+## Content
+
+We love getting contributions from anyone.  But keep in mind that Open
+MPI is used in production environments all around the world.
+
+If you're contributing a small bug fix, awesome!
+
+If you're contributing a large new piece of functionality, that will
+be best viewed if you -- or someone, anyone -- is also stepping up to
+help maintain that functionality over time.  We love new ideas and new
+features, but we do need to be realistic in what we can reliably test
+and deliver to our users.
+
+## Contributor's Declaration
+
+In order to ensure that we can keep distributing Open MPI under our
+[open source license](LICENSE), we need to ensure that all
+contributions are compatible with that license.
+
+To that end, we require that all Git commits contributed to Open MPI
+have a "Signed-off-by" token indicating that the commit author agrees
+with [Open MPI's Contributor's
+Declaration](https://github.com/open-mpi/ompi/wiki/Admistrative-rules#contributors-declaration).
+
+If you have not already done so, please ensure that:
+
+1. Every commit contains exactly the "Signed-off-by" token.  You can
+add this token via `git commit -s`.
+1. The email address after "Signed-off-by" must match the Git commit
+email address.
+
+## Style
+
+There are a small number of style rules for Open MPI:
+
+1. For all code:
+    * 4 space tabs.  No more, no less.
+    * No tab characters *at all*.  2 indentations are 8 spaces -- not a tab.
+    * m4 code is a bit weird in terms of indentation: we don't have a
+      good, consistent indentation style in our existing code.  But
+      still: no tab characters at all.
+1. For C code:
+    * We prefer if all blocks are enclosed in `{}` (even 1-line
+      blocks).
+    * We prefer that if you are testing equality with a constant, put
+      the constant on the *left* of the `==`.  E.g., `if (NULL ==
+      ptr)`.
+    * If there are no parameters to a C function, declare it with
+      `(void)` (vs. `()`).
+
+That's about it!