flesh out the README

README.md

@@ -23,3 +23,39 @@ export R_PREFIX="$HOME/.local"
 export PATH="$R_PREFIX/lib/ry/current/bin:$PATH"
 . "$R_PREFIX/lib/ry.bash_completion"
 ```
+
+## Usage
+
+Ry is a bit different from [other][rvm] [version][rbenv] [managers][n].  The major design goal of ry is to be explicit, unobtrusive, and easy to query.  For example, here's how you create a new installation:
+
+[rvm]: http://rvm.beginrescueend.com/
+[rbenv]: https://github.com/sstephenson/rbenv
+[n]: https://github.com/visionmedia/n
+
+``` bash
+ry install mri-1.9.3 http://ftp.ruby-lang.org/pub/ruby/1.9/ruby-1.9.3-p125.tar.gz
+```
+
+This creates an installation of Ruby called `mri-1.9.3` using the tarball from `ruby-lang.org`.  To switch to this ruby, use
+
+``` bash
+ry use mri-1.9.3 # or: ry mry-1.9.3
+```
+
+Want to use the latest HEAD?
+
+``` bash
+ry install mri-head https://github.com/ruby/ruby/tarball/HEAD
+```
+
+Or a custom commit?
+
+``` bash
+ry install mri-custom https://github.com/ruby/ruby/tarball/<hash>
+```
+
+It will also build Rubinius:
+
+``` bash
+ry install rbx https://github.com/rubinius/rubinius/tarball/v0.9.0
+```