documented new asset configuration item
documented remove_html_comments and relative_protocol_replacement config keys; fixed typo
Adds Mandrill email driver docs
Adds note for additional dependency

Shouldn't `mandril/mandrill` be added to the `suggest` part of `somposer.json`?
Merge pull request #646 from sagikazarmark/patch-1

Adds Mandrill email driver docs
本家差分をmerge