Add CONTRIBUTING.md file

CONTRIBUTING.md

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+# Contributing
+
+Bug reports and enhancements to the library are really valued and appreciated!
+
+# Bug reports
+
+If you find a bug, please report it! Bugs on the library are taken seriously 
+and a patch for them is usually pushed on the same day. 
+
+When reporting a bug, please make sure to indicate the platform (e.g. GNU/Linux, Windows, OSX)
+in which you came across the issue, as this is essential to finding the cause.
+
+## Packet parsing bugs
+
+If you find a bug related to packet parsing (e.g. a field on a packet contains an 
+invalid value), please try to provide a pcap file that contains the packet that
+was incorrectly parsed. Doing this will make it very simple to find the issue, plus
+you will be asked to provide this file anyway, so this just makes things
+easier.
+
+# Pull requests
+
+Pull requests are very welcomed. When doing a pull request please:
+
+* Base your PR branch on the `develop` branch. This is **almost always** pointing to the
+same commit as `master`, so you shouldn't have any issues changing the destination branch
+to `develop` at the time you try to do the pull request if you based your code on `master`.
+* Your code will be compiled and tests will be run automatically by the travis and 
+appveyor CI tools. If your code has issues on any of the tested platforms (GNU/Linux, Windows
+and OSX), please fix it or otherwise the PR won't be merged.