Add RESTclient layer

It has a nice interface for doing work with APIs and whatnot

> This is a tool to manually explore and test HTTP REST webservices. Runs queries from a plain-text query sheet, > displays results as a pretty-printed XML, JSON and even images.

[Package Link](https://github.com/pashky/restclient.el)
I like how you decoupled the leader from the chose key but for documentation consistency I prefer to replace `Evil-leader key` heading by `Key bindings` and `mc` by `<kbd>SPC m c</kbd>`.

Examples:
- heading: https://github.com/syl20bnr/spacemacs/commit/0de0b989b7f2ee3fe9d387a732fe0c7f4989a91d#diff-dc89fdcc242c31d756fdf63cc2b6a434R560
- key: https://github.com/syl20bnr/spacemacs/commit/0de0b989b7f2ee3fe9d387a732fe0c7f4989a91d#diff-dc89fdcc242c31d756fdf63cc2b6a434R562