minor symfony#8136 Add typescript loader documentation (davidmpaz, javiereguiluz)

This PR was submitted for the master branch but it was merged into the 3.3 branch instead (closes symfony#8136).

Discussion
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Add typescript loader documentation

This adds Encore TypeScript loader documentation.

@weaverryan sorry for taking long on this.....feel free to add other point you think is necessary.

Commits
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0e9908c Minor change
5b74bdd Add reference to docs in frontend page
d15e5ed Add typescript loader documentation

Author: xabbuh

frontend.rst

@@ -43,6 +43,7 @@ Adding more Features
 * :doc:`Enabling Vue.js (vue-loader) </frontend/encore/vuejs>`
 * :doc:`Configuring Babel </frontend/encore/babel>`
 * :doc:`Source maps </frontend/encore/sourcemaps>`
+* :doc:`Enabling TypeScript (ts-loader) </frontend/encore/typescript>`
 
 Optimizing
 ..........

frontend/encore/typescript.rst

@@ -0,0 +1,46 @@
+Enabling TypeScript (ts-loader)
+===============================
+
+Want to use `TypeScript`_? No problem! First, install the dependencies:
+
+.. code-block:: terminal
+
+    $ yarn add --dev typescript ts-loader
+
+Then, activate the ``ts-loader`` in ``webpack.config.js``:
+
+.. code-block:: diff
+
+    // webpack.config.js
+    // ...
+
+    Encore
+        // ...
+        .addEntry('main', './assets/main.ts')
+
+        .enableTypeScriptLoader()
+    ;
+
+That's it! Any ``.ts`` files that you require will be processed correctly. You can
+also configure the `ts-loader options`_ via a callback:
+
+.. code-block:: javascript
+
+    .enableTypeScriptLoader(function (typeScriptConfigOptions) {
+        typeScriptConfigOptions.transpileOnly = true;
+        typeScriptConfigOptions.configFileName = '/path/to/tsconfig.json';
+    });
+
+If React assets are enabled (``.enableReactPreset()``), any ``.tsx`` file will be
+processed as well by ``ts-loader``.
+
+Loader usage can be checked better in its `README`_ documentation.
+
+    "Use webpack like normal, including ``webpack --watch`` and ``webpack-dev-server``,
+    or through another build system using the Node.js API."
+
+    -- Running section of ts-loader documentation
+
+.. _`TypeScript`: https://www.typescriptlang.org/
+.. _`ts-loader options`: https://github.com/TypeStrong/ts-loader#options
+.. _`README`: https://github.com/TypeStrong/ts-loader#typescript-loader-for-webpack