Add developer guide (#57)

Signed-off-by: Rushi Agrawal <[email protected]>

DEVELOPER_GUIDE.md

@@ -0,0 +1,22 @@
+Run the linter and test suite to ensure your changes do not break existing code:
+
+```
+# Install Nox for task management
+$ python -m pip install nox
+
+# Auto-format and lint your changes
+$ nox -rs format
+
+# Run the test suite
+$ nox -rs test
+```
+
+To run the integration tests locally, run:
+
+```
+./.ci/run-tests opensearch true
+```
+
+The first argument, `opensearch` tells the server type to run integration test against. Valid values are `opensearch` and `opendistro`. The second argument tells whether to run server with security plugin enabled or not.
+
+Note that integration tests require docker to be installed and running, and downloads quite a bit of data from over the internet and hence take few minutes to complete.