Add documentation for the rb-extension-pack repository.

People were a bit confused about the purpose of rb-extension-pack and it
was unclear which extensions were usable and which weren't. To fix this,
I'm adding a series of READMEs, plus a CONTRIBUTING file for GitHub, to
make it clear what the status and requirements are for each extension.

Going forward, every extension should provide a README in the same
format.

CONTRIBUTING

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+If you're looking to contribute to an extension, we've written up a handy
+little guide that will be useful to you:
+
+http://www.reviewboard.org/docs/codebase/dev/contributing-patches/
+
+You do not need to clone this repository to produce new extensions. We just use
+it as a placeholder for any production or sample extensions we create.
+
+Please note that we do *not* accept pull requests! Instead, every patch must
+go through a code review process on our own Review Board server at
+http://reviews.reviewboard.org/.

README.md

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+Review Board Extension Pack
+===========================
+
+This repository contains a variety of extensions for Review Board. Some of
+these are experimental and not complete, while others can be used on a
+production server.
+
+See the READMEs in each directory for an explanation of the extension, the
+versions of Review Board its compatible with, and the status of the extension.

rbseverity/README.md

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+Comment Severity Extension
+==========================
+
+Overview
+--------
+
+This extension provides support for setting the severity level of a comment
+when commenting on a diff or a file attachment.
+
+The comment dialog is extended to replace the `Save` button with a series
+of severity buttons. Clicking one of these buttons will save the comment at
+that severity level.
+
+Severities are shown alongside the comments in reviews and in e-mails, and
+can be altered in the review dialog.
+
+
+Requirements
+------------
+
+This extension requires Review Board 2.0 beta 1 or higher.
+
+
+Status
+------
+
+This extension is production-ready.

rbwebhooks/README.md

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+Webhooks Extension
+==================
+
+Overview
+--------
+
+This extension adds support for webhooks. These can be used to notify a
+service when a review request is published by way of an HTTP request
+with a JSON payload containing information on the review request.
+
+
+Requirements
+------------
+
+This extension requires Review Board 1.7.x or higher.
+
+
+Status
+------
+
+This extension is currently unmaintained and is missing several features.
+Contributions are welcome.

rbxmlreview/README.md

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+XML Review Extension
+====================
+
+Overview
+--------
+
+This is a sample extension for providing a simple review UI for XML files.
+It was a student project as part of [UCOSP](http://ucosp.ca/).
+
+The extension is a work-in-progress and is not meant for use.
+
+
+Requirements
+------------
+
+This extension requires Review Board 1.7.x or higher.
+
+
+Status
+------
+
+This extension is currently unmaintained and not yet functional. Contributions
+are welcome.