doc: add new package

MONOREPO.md

@@ -65,3 +65,19 @@ The `release` script will automatically perform the tasks for all packages:
 - Check lint (tslint and prettier) issues
 
 If all steps are successful, it prompts you to publish packages into npm repository.
+
+### Add a package
+
+To add a new package, create a folder in [`packages`](packages) as the module folder, it is 
+like a common npm module with configuration files and your source code.
+
+We have some configuration files at the top level:
+
+- `.gitignore`
+- `.prettierignore`
+- `.nycrc.yml`
+
+For the consistency among all packages, no need to add them at package level unless there is a difference.
+
+Also add the new package and your name as its owner in [CODEOWNERS](CODEOWNERS). 
+And add the new module in the 'packages' list in [MONOREPO#packages](MONOREPO#packages).