CONTRIBUTING.md: add content

CONTRIBUTING.md

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+# CONTRIBUTING
+
+If you find an issue that interests you, please leave a note asking about it
+first. If you don't see that anyone has inquired about it, you can simply
+leave a note saying you are going to work on it. If people work on an issue
+without saying they are working on it, the result is sometimes three people
+submit a PR for the same issue.
+
+If you find a problem for which no ticket has yet been created, please don't
+hesitate to open a new ticket, and let us know if you are going to work on
+that issue.
+
+Please leave another note if you change your mind or if you get busy with other
+things and are unable to finish it. That lets me and other people know the
+ticket is available to be worked on by other people.
+
+## Coding style
+Please match the existing style
+
+Sometimes a patch will be a single line in a single file; other times a single
+patch will consist of changes to several files. Keep unrelated patches separate
+from each other (i.e. a separate PR for each patch).
+
+## Pull Requests
+1. [Fork the repo][fork] (if you haven't already done so)
+
+2. Clone it to your computer
+
+3. When you're ready to work on an issue, be sure you're on the **master** branch. From there,
+[create a separate branch][sep_branch] (e.g. issue_32)
+
+4. Make your changes. If you're unsure of some details while you're making edits, you can
+discuss them on the ticket.
+
+5. Commit your changes. [git-cola](https://git-cola.github.io/) is a
+nice GUI front-end for adding files and entering commit messages
+(git-cola is probably available from your OS repository).
+
+6. Push the working branch (e.g. issue_32) to your remote fork and make your
+[pull request][PR]
+    * Do not merge it with the master branch on your fork. That would result in multiple, or
+    unrelated patches being included in a single PR.
+
+7. If any further changes need to be made, comments will be made on the pull request.
+
+It's possible to work on two or more different patches (and therefore multiple branches) at
+one time, but it's recommended that beginners only work on one patch at a time.
+
+### Syncing ###
+Periodically, especially before starting a new patch, you'll need the sync your
+repo with the remote upstream. GitHub has instructions for doing this:
+
+1. [Configuring a remote for a fork](https://help.github.com/articles/configuring-a-remote-for-a-fork/)
+    * For step 3 on that page, use https://github.com/sexybiggetje/pixdisp for the URL.
+
+2. [Syncing a Fork](https://help.github.com/articles/syncing-a-fork/)
+    * On that page, it shows how to merge the **master** branch (steps 4 & 5 of **Syncing a Fork**).
+
+[fork]: https://github.com/sexybiggetje/pixdisp/fork
+[sep_branch]: https://github.com/Kunena/Kunena-Forum/wiki/Create-a-new-branch-with-git-and-manage-branches
+[PR]: https://help.github.com/articles/creating-a-pull-request-from-a-fork/