Improve README

README.md

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 # Adversarial collective sensing game
+
+The adversarial collective sensing game is implemented in JavaScript
+and must be compiled to create bundle.js
+before being served by a web server.
+
+## Installing dev tools
+
+Make sure you have Node.js >= version 6.
+
+First [install `yarn`](https://yarnpkg.com/en/docs/install).
+(`yarn` is similar to `npm`, but better,
+and uses the same package repository.)
+
+Next run `yarn` to install the dependencies:
+
+    $ yarn
+
+## Building bundle.js
+
+To update bundle.js:
+
+    $ yarn run bundle
+
+Note: Updates to the bundle should be committed to version control.
+
+## Managing dependencies
+
+To add a new dependency, use the `yarn add` command:
+
+    $ yarn add snargle-fraster
+
+If it's a dependency only at build time, use `yarn add [pkg] --dev`.
+
+This updates the `yarn.lock` file, which records the specific
+releases which were used. Other developers will get those
+releases when they run `yarn`. Updates to `yarn.lock`
+should be committed to version control.