Adding information on BWC tests in Developer guide

Signed-off-by: Varun Jain <[email protected]>

DEVELOPER_GUIDE.md

@@ -11,6 +11,8 @@
     - [Run Single-node Cluster Locally](#run-single-node-cluster-locally)
     - [Run Multi-node Cluster Locally](#run-multi-node-cluster-locally)
   - [Debugging](#debugging)
+  - [Backwards Compatibility Testing](#backwards-compatibility-testing)
+    - [Adding new tests](#adding-new-tests)
   - [Supported configurations](#supported-configurations)
   - [Submitting Changes](#submitting-changes)
 
@@ -184,6 +186,25 @@ Additionally, it is possible to attach one debugger to the cluster JVM and anoth
 ./gradlew :integTest -Dtest.debug=1 -Dcluster.debug=1
 ```
 
+## Backwards Compatibility Testing
+
+The purpose of Backwards Compatibility Testing and different types of BWC tests are explained [here](https://github.com/opensearch-project/opensearch-plugins/blob/main/TESTING.md#backwards-compatibility-testing)
+
+Use these commands to run BWC tests for k-NN:
+1. Rolling upgrade tests: `./gradlew :qa:rolling-upgrade:testRollingUpgrade`
+2. Full restart upgrade tests: `./gradlew :qa:restart-upgrade:testAgainstNewCluster`
+3. `./gradlew :qa:bwcTestSuite` is used to run all the above bwc tests together.
+
+Use this command to run BWC tests for a given Backwards Compatibility Version:
+```
+./gradlew :qa:bwcTestSuite -Dbwc.version=2.9.0
+```
+Here, we are testing BWC Tests with BWC version of plugin as 2.9.0.
+
+### Adding new tests
+
+Before adding any new tests to Backward Compatibility Tests, we should be aware that the tests in BWC are not independent. While creating an index, a test cannot use the same index name if it is already used in other tests.
+
 ### Supported configurations
 
 By default, neural-search plugin supports `lucene` k-NN engine for local runs. Below is the sample request for creating of new index using this engine: