Add plugin development notes in a readme. (#209) (#213)

* Add plugin development notes in a readme.

* Update README.md

Co-authored-by: Cas Donoghue <[email protected]>

* Update README.md

Co-authored-by: Cas Donoghue <[email protected]>

---------

Co-authored-by: Cas Donoghue <[email protected]>
(cherry picked from commit f48b08b)

Co-authored-by: Mashhur <[email protected]>

Author: github-actions[bot]

README.md

@@ -7,4 +7,73 @@ It can be used in a Logstash pipeline to perform the transformations that are ap
 ## Documentation
 
 Logstash provides infrastructure to automatically generate documentation for this plugin from its [asciidoc source](docs/index.asciidoc).
-All plugin documentation are placed under one [central location](http://www.elastic.co/guide/en/logstash/current/).
+All plugin documentation are placed under one [central location](http://www.elastic.co/guide/en/logstash/current/).
+
+## Development
+### Prerequisites
+- Clone the [plugin repo](https://github.com/elastic/logstash-filter-elastic_integration)
+- Check out the branch you would like to build, such as `8.x` or `9.x/main`, etc...
+  - Note that, `8.series` branches require JDK17 and `9.series`/`main` requires JDK21.
+- Set the Logstash source path in the `gradle.properties` file. Open the file and update the `LOGSTASH_PATH`.
+- Make sure Logstash is compiled.
+- Point to the Elasticsearch branch/version to build the plugin with. Open the `gradle.properties` file and update `ELASTICSEARCH_TREEISH`.
+
+### Build
+We use Gradle tool to build this plugin. Gradle is configured to load the Logstash core jars from the `LOGSTASH_PATH` path, download the Elasticsearch version defined with `ELASTICSEARCH_TREEISH` and build the plugin.
+The build process may fail if the Elasticsearch interfaces the plugin is using have changed.
+
+The following command builds plugin and generates the jar file, can be locally installed in Logstash core and verified:
+```shell
+./gradlew clean vendor localGem
+```
+
+### Plugin test
+The recommendation to run tests to align on our common [`plugins/.ci`](https://github.com/logstash-plugins/.ci) tool.
+Run the following command under the plugin source dir to download the CI tool:
+```shell
+mkdir -p .ci-test && curl -sL --retry 5 --retry-delay 5 https://github.com/logstash-plugins/.ci/archive/1.x.tar.gz | tar zxvf - --keep-old-files --strip-components=1 -C .ci-test
+```
+
+Export environment variables:
+```shell
+SNAPSHOT=false                # set to true if targeting the SNAPSHOT
+ELASTIC_STACK_VERSION=8.17.0  # stack version to run the plugin on
+INTEGRATION=false             # set to true if you want integration tests to run, it also requires SECURE_INTEGRATION=true
+```
+
+Run tests on the local docker environment, make sure your docker-engine is running.
+```shell
+./.ci/docker-setup.sh && ./.ci/docker-run.sh
+```
+
+### Running your unpublished plugin in Logstash
+
+#### Run in a local Logstash clone
+- Edit Logstash `Gemfile` and add the local plugin path, for example:
+```ruby
+gem "logstash-filter-awesome", :path => "/your/local/logstash-filter-awesome"
+```
+- Install plugin
+```sh
+bin/logstash-plugin install --no-verify
+```
+- Run Logstash with your plugin
+```sh
+bin/logstash -e 'filter {awesome {}}'
+```
+At this point any modifications to the plugin code will be applied to this local Logstash setup. After modifying the plugin, simply rerun Logstash.
+
+#### Run in an installed Logstash
+- Edit Logstash `Gemfile` and add the local plugin path, for example:
+```ruby
+gem "logstash-filter-awesome", :path => "/your/local/logstash-filter-awesome"
+```
+- Build your plugin gem
+```sh
+./gradlew clean vendor localGem
+```
+- Install the plugin from the Logstash home
+```sh
+bin/logstash-plugin install --no-verify
+```
+- Start Logstash and proceed to test the plugin