Add tldrb

Hey, you said to let y'all know if I'm building a client, so here y'all go.

It's pretty much built already, in the sense that it'll grab a tldr (tries OS-specific first, then "common" if there's no OS-specific one or the OS isn't supported yet; by the way, us BSD users feel rather left out :) ).  Doesn't try to format anything yet, so it just barfs out raw markdown; if I keep tinkering with this, the next addition will probably be a basic formatter.

README.md

@@ -28,6 +28,7 @@ You can access these pages on your computer using one of the following clients:
 - [Exilir client](https://github.com/edgurgel/tldr): binaries available soon
 - [C++ client](https://github.com/tldr-pages/tldr-cpp-client): `brew tap tldr-pages/tldr && brew install tldr`
 - [Android client](https://github.com/gianasista/tldr-viewer): available on [Google Play](https://play.google.com/store/apps/details?id=de.gianasista.tldr_viewer)
+- [Ruby client](https://github.com/YellowApple/tldrb): `gem install tldrb`
 - [Web client](https://github.com/ostera/tldr.js): try tldr on your browser [here](https://ostera.github.io/tldr.js)!
 
 Let us know if you are building one and we can add it to this list!