Update contributing file

CONTRIBUTING.md

@@ -11,6 +11,52 @@ Paho reflects the inherent physical and cost constraints of device connectivity.
 
 - https://projects.eclipse.org/projects/technology.paho
 
+Source
+------
+
+The Paho Python code is stored in a git repository. The URLs to access it are:
+
+ssh://<username>@git.eclipse.org:29418/paho/org.eclipse.paho.mqtt.python
+https://<username>@git.eclipse.org/r/paho/org.eclipse.paho.mqtt.python
+
+A web browsable repository is available at
+
+http://git.eclipse.org/c/paho/org.eclipse.paho.mqtt.python.git
+
+Contributing a patch
+--------------------
+
+The Paho repositories are accessed through Gerrit, the code review
+project, which makes it possible for anybody to clone the repository, make
+changes and push them back for review and eventual acceptance into the project.
+
+To do this, you must follow a few steps. The first of these are described at
+
+- https://wiki.eclipse.org/Development_Resources/Contributing_via_Git
+
+* Sign the Eclipse CLA
+* Use a valid commit record, including a signed-off-by entry.
+
+There are further details at
+
+- https://wiki.eclipse.org/Development_Resources/Handling_Git_Contributions
+
+Once the patch is pushed back to Gerrit, the project committers will be
+informed and they will undertake a review of the code. The patch may need
+modifying for some reason. In order to make amending commits more
+straightforward, the steps at
+https://git.eclipse.org/r/Documentation/cmd-hook-commit-msg.html should be
+followed. This automatically inserts a "Change-Id" entry to your commit message
+which allows you to amend commits and have Gerrit track them as the same
+change.
+
+What happens next depends on the content of the patch. If it is 100% authored
+by the contributor and is less than 250 lines (and meets the needs of the
+project), then it can be committed to the main repository. If not, more steps
+are required. These are detailed in the legal process poster:
+
+- http://www.eclipse.org/legal/EclipseLegalProcessPoster.pdf
+
 Developer resources:
 --------------------