Moved sentence on non-invasiveness from the usage section to the introduction, to avoid the first sentence in the "Usage" section telling us about not using the bundle.
Revised the introduction of the "Usage" section.
Merge branch 'master' of github.com:webfactory/icu-translation-bundle
Revised the "Placeholder Substitution" section.
Revised the "Gender Specific Translation" section.
Revised the "Pluralization" section.