RULEAPI-701: document the use of the new "Add language to a rule" GH action (#438)

README.adoc

@@ -90,28 +90,35 @@ Otherwise, you can use https://asciidoclive.com/[AsciiDocLIVE] and this https://
 
 ==== To add language to an existing rule
 
-If the rule exists on the main branch, create a PR named "Create rule Sxxxx[_language_]: _rule title_". For example:
+Similar to adding a new rule, if the rule exists on the main branch, trigger the "Add language to a rule" GitHub action.
+
+. go to the https://github.com/SonarSource/rspec/actions/workflows/add_language.yml[Add language to a rule] GitHub action
+. click on the grey _Run wokflow_ button (on the right).
+. in the field _"ID of an existing rule"_ write the ID of the existing rule you want to add language to.
+It must be in a form `Sxxxx` where `xxxx` is the number of the rule.
+For example, `S100`, `S1234`, `S6000`.
+. write the list of languages you want to specify this rule for.
+. in the field _"Language to be added to the rule"_ specify one language to be added to the rule. +
+It can be any of the following:
+include::supported_languages.adoc[]
+. click on the green _Run workflow_ button.
+
+image::img/add-language-workflow.png[]
+
+You should see https://github.com/pulls/assigned[a new pull request assigned to you].
+It might take up to a few minutes to appear.
+It contains a scaffolding of files for the new rule. Feel free to modify it as you please.
+
+The title of the PR for a new rule will say only "Create rule Sxxxx[lang]", which is not very informative. +
+Modify the title to better summarize the nature or the rule, so that it is easier to find when searching through unimplemented rules.
+Do preserve the "Create rule Sxxxx[lang]" prefix, as it is used by our tooling.
+For example:
 ----
 Create rule S100[java]: Method names should comply with a naming convention
 ----
 
 Otherwise, if the rule has not been merged yet (i.e. the rule has not been implemented by any plugin), you can use the already existing PR corresponding to this rule. +
 
-Then add a corresponding github label to the PR.
-
-In order to automatically trigger a preview of the rule specialization in the deployment, your branch name must start with `rule/`.
-
-Then,
-
-* Create a subdirectory with the name of the language you want to add. +
-It can be any of the following:
-include::supported_languages.adoc[]
-* Add two files in this subdirectory:
-
-** `rule.adoc`: write down the whole description of the rule. If you want to include already existing parts (that are in the current or the parent directory), use the `include::` syntax.
-** `metadata.json`: it automatically inherits from the `metadata.json` of the parent directory so you only need to write the parts that need to be overwritten. +
-Please note that in any case this file should at least contain two curly braces (`{ }`)
-
 ==== To modify an existing rule
 Create a branch for your modifications manually.
 Then open a pull request manually.