Add instructions for building and testing MuJS

tools/mujs/README.md

@@ -4,3 +4,29 @@ This directory contains Mu's JavaScript compiler.
 
 It implements a subset of JavaScript with TypeScript-style type annotations, and can compile that subset into MuPack/IL.
 
+## Building and Testing
+
+MuJS is built independent from the overall Mu toolchain.  First clone and cd to the right place:
+
+    $ git clone git@github.com:marapongo/mu
+    $ cd mu/tools/mujs
+
+Next, install dependencies, ideally [using Yarn](https://yarnpkg.com/en/docs/install):
+
+    $ yarn install
+
+(NPM can be used instead, but Yarn offers better performance, reliability, and security, so it's what we use below.)
+
+From there, to build:
+
+    $ yarn run build
+
+It's possible to simply run the TypeScript compiler using `tsc`, however the Yarn build step performs a couple extra
+steps; namely, it runs TSLint and also copies some test baseline files into the right place.
+
+Next, to test, simply run:
+
+    $ yarn run test
+
+It will be obvious if the tests passed or failed and, afterwards, code coverage data will be output to the console.
+