Clarify readme instructions for developers

Setting up the SDK for development, I noticed that 3 tests failed on
develop branch. Linking the JS SDK made the tests pass. It looks like
the assumption is that developers link against the develop branch
of JS SDK to develop the React SDK.

Clarify this in the readme for completeness sake.

Signed-off-by: Jason Robinson <jasonr@matrix.org>

README.md

@@ -127,3 +127,36 @@ Github Issues
 
 All issues should be filed under https://github.com/vector-im/riot-web/issues
 for now.
+
+Development
+===========
+
+Ensure you have the latest stable Node JS runtime installed. Then check out
+the code and pull in dependencies:
+
+```bash
+git clone https://github.com/matrix-org/matrix-react-sdk.git
+cd matrix-react-sdk
+git checkout develop
+npm install
+```
+
+`matrix-react-sdk` depends on `matrix-js-sdk`. To make use of changes in the 
+latter and to ensure tests run against the develop branch of `matrix-js-sdk`,
+you should run the following which will sync changes from the JS sdk here.
+
+```bash
+npm link ../matrix-js-sdk
+```
+
+Command assumes a checked out and installed `matrix-js-sdk` folder in parent
+folder.
+
+Running tests
+=============
+
+Ensure you've followed the above development instructions and then:
+
+```bash
+npm run test
+```