Add a contributing guidelines file.

CONTRIBUTING.md

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+# Contributing to OpenMVG
+
+First off all, thanks for taking the time to contribute!
+
+**Contributions** (bug reports or bug fixes, features, etc.) and **feedback**  are welcome and should be submitted thanks to issues form and/or pull requests on GitHub.
+
+
+
+Here the list of things to check for contribution:
+
+1. Start your dev from **develop** branch:
+
+  -  Master is storing the current release.
+  - Develop store the WIP for the next release candidate. Please submit any new features to the develop branch.
+
+2. Consider adding unit test:
+
+  - Add unit tests for all newly added code.
+  - It helps to check accuracy of your code implementation and show how to use the API. It helps also any future maintenance.
+
+2. Consider adding documentation:
+
+  - Please use doxygen documentation.
+
+     - You can also consider extending the exiting Sphinx documentation (see ./docs/sphinx)
+
+
+4. Push your change and make a PR towards develop:
+
+  - please check the status of the continuous integration tools:
+      -  Compilation (travis, appveyor),
+      -  Code quality and style (codacity)).
+
+If you want to learn more about OpenSource we refer you to this:
+https://opensource.guide/