Update documentation with new filters from rocket-lazyload-common

[nicomollet]

Is your feature request related to a problem? Please describe.
Filters coming from rocket-lazyload-common are not documented in the readme files of rocket-lazy-load.

Describe the solution you'd like
The new filter rocket_lazyload_noscript needs documentation.

And the older filters are not documented in the readme files.

• rocket_lazyload_placeholder
• rocket_lazyload_exclude_youtube_thumbnail
• rocket_lazyload_html
• rocket_lazyload_placeholder