Add a contributing guidelines section

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I have updated all branches to be inline with the latest release (10.2.0). We need a strategy for maintaining the quality of the branches, ensuring that they are always inline with the latest project version (or that they become deprecated). Here is my proposed strategy:

• Have a next branch which lines up all changes to become a new master version.
• When we are happy with the state of next and are ready for a push we start a PR update process of all existing branches, merging in the changes from next.
• Then we can deploy the new version side by side to accepting all PRs.