Add documentation about the frontend build system

Author: Kobzol

site/frontend/README.md

@@ -1,7 +1,7 @@
 # `rustc-perf` site frontend
 This directory contains code for the `rustc-perf` website frontend.
 
-## Build
+## Build instructions
 - Install dependencies
    ```console
    $ npm install
@@ -17,3 +17,12 @@ This directory contains code for the `rustc-perf` website frontend.
 
 The files are packaged into the `dist` directory, from which they are embedded into the `rustc-perf`
 binary.
+
+## Build system
+The frontend is built using the `Parcel` bundler. It generates several JavaScript files (one for each
+page in the perf.RLO website) into the `dist` directory.
+
+It uses a custom backend for transpiling `TypeScript`, which transpiles for browsers defined in
+the `browserslist` attribute in `package.json`. It does not perform type-checking, that is why
+there is a dedicated `npm run check` action that uses `tsc`. It is performed on CI to find typ errors.
+The `tsconfig.json` file is only used by the type-checking action.

site/frontend/package-lock.json

@@ -26,5 +26,6 @@
       "source": "src/pages/dashboard.ts",
       "distDir": "dist/scripts"
     }
-  }
+  },
+  "browserslist": "> 0.5%, last 3 years, not dead"
 }

site/frontend/package.json

@@ -1,7 +1,8 @@
 {
   "compilerOptions": {
     "lib": ["es5", "es2015", "dom"],
-    "target": "es5",
+    "target": "es2017",
+    "moduleResolution": "node",
     "rootDir": "src"
   }
 }