Update CONTRIBUTING.md to include building JS (#9454)

* Update CONTRIBUTING.md to include building JS

Signed-off-by: sunboyy <sura_sun@hotmail.com>

* Edit minimum Node.js version in CONTRIBUTING.md

Signed-off-by: sunboyy <sura_sun@hotmail.com>

Co-authored-by: Antoine GIRARD <sapk@users.noreply.github.com>
Co-authored-by: techknowlogick <matti@mdranta.net>
Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: zeripath <art27@cantab.net>

CONTRIBUTING.md

@@ -114,10 +114,10 @@ included in the next released version.
 ## Building Gitea
 
 Generally, the go build tools are installed as-needed in the `Makefile`.
-An exception are the tools to build the CSS and images.
+An exception are the tools to build the CSS, JS and images.
 
-- To build CSS: Install [Node.js](https://nodejs.org/en/download/package-manager) at version 8.0 or above
+- To build CSS and JS: Install [Node.js](https://nodejs.org/en/download/package-manager) at version 10.0 or above
-  with `npm` and then run `npm install` and `make css`.
+  with `npm` and then run `npm install`, `make css` and `make js`.
 - To build Images: ImageMagick, inkscape and zopflipng binaries must be
   available in your `PATH` to run `make generate-images`.