Added 'Environemnt variables' cookbook page

docs/in-depth/cookbook/environment-variables.md

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+# Managing deploys with environment variables
+
+Environment variables provide a more streamlined way to manage your build process. We can also use it to define "build toggles", or environment-based variables that will be injected into your scripts to be used during runtime.
+
+## Setting it up
+
+Let's say that we want to set `NODE_ENV` to `production` for uploading to our main branch, and `development` for uploading to our Simulation branch. First we'll catch the environment variable and assign the compile target.
+
+```js
+// rollup.config.js
+
+const isProduction = process.env.NODE_ENV === 'production'
+
+// use the `main` target from `screeps.json` in production mode
+const cfg = isProduction ? 'main' : 'sim';
+
+export default {
+  // ...
+  plugins: [
+    // ...
+    screeps({
+      config: require("./screeps")[cfg],
+      // if `NODE_ENV` is local, perform a dry run
+      dryRun: process.env.NODE_ENV === 'local'
+    })
+  ]
+}
+```
+
+## Running a deploy
+
+Then we'll change the build tasks on `package.json` to pass the environment variable before running the rollup command.
+
+```json
+{
+  "tasks": {
+    "deploy-prod": "NODE_ENV=production rollup -c",
+    "deploy-dev": "NODE_ENV=development rollup -c",
+  }
+}
+```
+
+**Note:** On Windows, setting the environment variables as defined above will not work. For a cross-platform solution to define environment variables, use `cross-env`.
+
+```bash
+npm install --save-dev cross-env
+```
+```json
+{
+  "tasks": {
+    "deploy-prod": "cross-env NODE_ENV=production rollup -c",
+    "deploy-dev": "cross-env NODE_ENV=development rollup -c",
+  }
+}
+```
+
+Now let's give it a try! Run `npm run deploy-dev` or `npm run deploy-prod` and see if your code is uploaded properly.
+
+## Setting up build toggles
+
+[TODO]