Add CONTRIBUTING.md

Author: jedbrown

CONTRIBUTING.md

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+# How to contribute
+
+Contributions to libCEED are welcome.
+
+Please use a pull request to the appropriate branch ('maint' for
+backward-compatible bug fixes for the last stable release, 'master' for
+new features and everything else).
+
+Please make your commits well-organized and atomic, using `git rebase
+--interactive` as needed.  Open an issue or RFC pull request to discuss
+any significant changes before investing time.  Check that the tests
+pass using `make prove` or `make test`.  If adding a new feature, please
+add or extend a test so that it is tested.
+
+Write commit messages for a reviewer of your pull request and for a
+future developer (maybe you) that bisects and finds that a bug was
+introduced in your commit.  The assumptions that are clear in your head
+right now are likely not.  Give credit where credit is due using tags
+such as `Reported-by: Helpful User <[email protected]>`.  Please use a
+real name and email for your author information (`git config user.name`
+and `user.email`).
+
+Please avoid "merging from upstream" (like merging 'master' into your
+feature branch) unless there is a specific reason to do so, in which
+case you should explain why in the merge commit.
+[Rationale](https://lwn.net/Articles/328436/) from
+[Junio](https://gitster.livejournal.com/42247.html) and
+[Linus](http://yarchive.net/comp/linux/git_merges_from_upstream.html).
+
+By submitting a pull request, you are affirming the following.
+
+## [Developer's Certificate of Origin 1.1](https://developercertificate.org/).
+
+By making a contribution to this project, I certify that:
+
+(a) The contribution was created in whole or in part by me and I
+    have the right to submit it under the open source license
+    indicated in the file; or
+
+(b) The contribution is based upon previous work that, to the best
+    of my knowledge, is covered under an appropriate open source
+    license and I have the right under that license to submit that
+    work with modifications, whether created in whole or in part
+    by me, under the same open source license (unless I am
+    permitted to submit under a different license), as indicated
+    in the file; or
+
+(c) The contribution was provided directly to me by some other
+    person who certified (a), (b) or (c) and I have not modified
+    it.
+
+(d) I understand and agree that this project and the contribution
+    are public and that a record of the contribution (including all
+    personal information I submit with it, including my sign-off) is
+    maintained indefinitely and may be redistributed consistent with
+    this project or the open source license(s) involved.
+
+