Extend README with an instruction for the local setup (#253)

Author: lutien

README.md

@@ -7,3 +7,31 @@ WebDriver BiDi is not ready. Here's what we have so far:
 - An [explainer](./explainer.md) with more background and goals
 - Detailed [proposals](./proposals/) for the initial protocol
 - A [unofficial spec draft](https://w3c.github.io/webdriver-bidi/) waiting to be fleshed out
+
+
+## How to build the specification locally
+
+We use [bikeshed](https://tabatkins.github.io/bikeshed/) to generate the specification.
+
+Make sure you have the [right version of python](https://tabatkins.github.io/bikeshed/#install-py3) installed.
+
+Now you can run in your terminal:
+```
+./scripts/build.sh --install
+```
+This script will install `bikeshed` and generate an `index.html` file for the specification.
+
+Later on, you can omit `--install` argument to skip the installation step.
+
+## How to generate CDDL locally
+
+Make sure you have [npm](https://docs.npmjs.com/downloading-and-installing-node-js-and-npm)
+and [rust](https://www.rust-lang.org/tools/install) installed.
+
+Now you can run in your terminal:
+```
+./scripts/test.sh --install
+```
+This script will install required npm and cargo packages and generate CDDL.
+
+Later on, you can omit `--install` argument to skip the installation step.