docs: add linked library story (#6292)

Blocked on #6276.

Author: filipesilva

docs/documentation/stories.md

@@ -26,3 +26,4 @@
  - [Application Environments](stories/application-environments)
  - [Autoprefixer Configuration](stories/autoprefixer)
  - [Deploy to GitHub Pages](stories/githug-pages)
+ - [Linked Library](stories/linked-library)

docs/documentation/stories/linked-library.md

@@ -0,0 +1,62 @@
+# Linked libraries
+
+While working on a library, it is common to use [npm link](https://docs.npmjs.com/cli/link) to
+avoid reinstalling the library on every build.
+
+While this is very useful there are a few caveats to keep in mind.
+
+## The library needs to be AOT compatible
+
+Angular CLI does static analysis even without the `--aot` flag in order to detect lazy-loade routes.
+If your library is not AOT compatible, you will likely get a static analysis error.
+
+## The library still needs to be rebuilt on every change
+
+Angular libraries are usually built using TypeScript and thus require to be built before they
+are published.
+For simple cases, a linked library might work even without a build step, but this is the exception
+rather than the norm.
+
+If a library is not being built using its own build step, then it is being compiled by the
+Angular CLI build system and there is no guarantee that it will be correctly built.
+Even if it works on development it might not work when deployed.
+
+When linking a library remember to have your build step running in watch mode and the library's
+`package.json` pointing at the correct entry points (e.g. 'main' should point at a `.js` file, not
+a `.ts` file).
+
+## Use TypesScript path mapping for Peer Dependencies
+
+Angular libraries should list all `@angular/*` dependencies as
+[Peer Dependencies](https://nodejs.org/en/blog/npm/peer-dependencies/).
+This insures that, when modules ask for Angular, they all get the exact same module.
+If a library lists `@angular/core` in `dependencies` instead of `peerDependencies` then it might
+get a *different* Angular module instead, which will cause your application to break.
+
+While developing a library, you'll need to have all of your peer dependencies also installed
+via `devDependencies` - otherwise you could not compile.
+A linked library will then have it's own set of Angular libraries that it is using for building,
+located in it's `node_modules` folder.
+This can cause problems while building or running your application.
+
+To get around this problem you can use the TypeScript
+[path mapping](https://www.typescriptlang.org/docs/handbook/module-resolution.html#path-mapping).
+With it, you can tell TypeScript that it should load some modules from a specific location.
+
+You should list all the peer dependencies that your library uses in `./tsconfig.json`, pointing
+them at the local copy in the apps `node_modules` folder.
+This ensures that you all will always load the local copies of the modules your library asks for.
+
+```
+{
+  "compilerOptions": {
+    // ...
+    // Note: these paths are relative to `baseUrl` path.
+    "paths": {
+      "@angular/*": [
+        "../node_modules/@angular/*"
+      ]
+    }
+  }
+}
+```