Added installation instructions to README

Fixes #6
Add reference to 'Install ValidForm Builder' in docs
Note about v3.0 changes
Only init custom js if needed

This prevents output like `var objForm = (typeof ValidForm !== "undefined") ? new ValidForm("validform_1718109020", "") : new ValidForm("validform_1718109020", "");`
Merge remote-tracking branch 'origin/master' into wip-new-documentation

Signed-off-by: Robin van Baalen <robin@neverwoods.com>

Conflicts:
	docs/documentation.md