Add documentation for japicmp tasks

Signed-off-by: Andriy Redko <[email protected]>

DEVELOPER_GUIDE.md

@@ -57,6 +57,7 @@
       - [Developer API](#developer-api)
       - [User API](#user-api)
       - [Experimental Development](#experimental-development)
+      - [API Compatibility Checks](#api-compatibility-checks)
     - [Backports](#backports)
     - [LineLint](#linelint)
     - [Lucene Snapshots](#lucene-snapshots)
@@ -607,6 +608,20 @@ a LTS feature but with additional guard rails and communication mechanisms to si
 release, or be removed altogether. Any Developer or User APIs implemented along with the experimental feature should be marked with `@ExperimentalApi` (or documented as
 `@opensearch.experimental`) annotation to signal the implementation is not subject to LTS and does not follow backwards compatibility guidelines.
 
+#### API Compatibility Checks
+
+The compatibility checks for public APIs are performed using [japicmp](https://siom79.github.io/japicmp/) and are available as separate Gradle tasks (those are run on demand at the moment):
+
+```
+./gradlew japicmp
+```
+
+By default, the API compatibility checks are run against the latest released version of the OpenSearch, however the target version to compare to could be provided using system property during the build, fe.:
+
+```
+./gradlew japicmp  -Djapicmp.compare.version=2.14.0-SNAPSHOT
+```
+
 ### Backports
 
 The Github workflow in [`backport.yml`](.github/workflows/backport.yml) creates backport PRs automatically when the original PR with an appropriate label `backport <backport-branch-name>` is merged to main with the backport workflow run successfully on the PR. For example, if a PR on main needs to be backported to `1.x` branch, add a label `backport 1.x` to the PR and make sure the backport workflow runs on the PR along with other checks. Once this PR is merged to main, the workflow will create a backport PR to the `1.x` branch.