Reference blogit-admin throughout the docs

[arnaudoff]

Hello,

I believe it would be great if you could reference the dependency on blogit-admin gem in the installation instructions in the README.md and the detailed docs, because I've never used the gem before and it was only after I spent some hours researching and reading the source code that I found it is required here.

Best wishes and thanks for the awesome gem!

[Bodacious]

Hi @arnaudoff

I'm sorry to have cost you so much time. Adding a note to the README now.