Add a contribution doc

CONTRIBUTING.md

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+# Submitting a contribution
+
+This repo is a repackage of code made available by Kevin Weiner at
+http://www.fmsware.com/stuff/gif.html - for full attribution of the original code,
+check the Java file headers.
+
+Where alterations have occurred since the initial import, those changes are
+licenced under the [Apache V2 licence](http://www.apache.org/licenses/LICENSE-2.0.html),
+and all submitted patches must be under tthat licence too.
+
+If you'd like to submit a patch to this project, please state clearly
+in your pull-request that the contribution is **your own work**, and either:
+
+a) Assign the copyright on the contribution to myself, Roberto Tyley
+
+**or**
+
+b) Disclaim copyright on it and thus put it in the public domain
+
+**Please specify which option you want to use when creating your pull request.**