Add documentation about using Qt Quick Compiler with qrc .js files

Application developer need to make sure that they skip processing .js
files by the Qt Quick Compiler, if the files will be loaded inside
HTML pages.

Task-number: QTBUG-66155
Change-Id: I09a3c6fa1f31b60a09d30012c6e15ae872bac639
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>