doc: add hints for contributors

Described how to setup necessary environment and regenerate the
resulting `index.js` file to ease a pathway of a first time contributor
like me.

Author: Totktonada

HACKING.md

@@ -0,0 +1,18 @@
+## First steps
+
+Clone the repository and install dependencies.
+
+```shell
+$ git clone [email protected]:tarantool/setup-tarantool.git
+$ cd setup-tarantool
+$ npm install
+```
+
+## Contribute
+
+Edit the source and regenerate the `index.js` file.
+
+```shell
+$ vim src/main.ts
+$ npm run pre-checkin
+```

README.md

@@ -42,3 +42,5 @@ The scripts and documentation in this project are released under the [MIT Licens
 # Contributions
 
 Contributions are welcome! Just open an issue or send a pull request.
+
+Check out [HACKING.md](./HACKING.md) file for hints.