Create CONTRIBUTING.md (#264)

CONTRIBUTING.md

@@ -0,0 +1,32 @@
+# Contributing to Dendrite
+
+Everyone is welcome to contribute to Dendrite! We aim to make it as easy as
+possible to get started.
+
+## Picking Things To Do
+
+If you're new then feel free to pick up an issue labelled [easy](https://github.com/matrix-org/dendrite/labels/easy).
+These should be well-contained, small pieces of work that can be picked up to
+help you get familiar with the code base.
+
+Once you're comfortable with hacking on Dendrite there are issues lablled as
+[help-wanted](https://github.com/matrix-org/dendrite/labels/help-wanted), these
+are often slightly larger or more complicated pieces of work but are hopefully
+nonetheless fairly well-contained.
+
+We ask people who are familiar with Dendrite to leave the [easy](https://github.com/matrix-org/dendrite/labels/easy)
+issues so that there is always a way for new people to come and get involved.
+
+## Getting Help
+
+For questions related to developing on Dendrite we have a dedicated room on
+Matrix [#dendrite-dev:matrix.org](https://riot.im/develop/#/room/#dendrite-dev:matrix.org)
+where we're happy to help.
+
+For more general questions please use [#dendrite:matrix.org](https://riot.im/develop/#/room/#dendrite:matrix.org).
+
+## Sign off
+
+We ask that everyone who contributes to the project signs off their
+contributions, in accordance with the [DCO](https://github.com/matrix-org/matrix-doc/blob/master/CONTRIBUTING.rst#sign-off).
+