Documentation on website and assembly.js header

[barockobamo]

From #26 :

put online the tool on the jscert website (DONE, needs below:)
- make sure to describe the supported features
(ecma5 minus for-in and a few other things)
- explain that currently JS files are generated
from ML files and thus should not be edited,
although this might change in the future.
- assembly.js should have a header
to tell people not to use this file

[IgnoredAmbience]

Ideally this content should also be part of the main repository documentation: use the build process to place it into web distribution. (for consistency)