added Bundler readme file

plugins/bundler/README.md

+# Bundler
+
+- adds completion for basic bundler commands
+- adds short aliases for common bundler commands
+  - `be` aliased to `bundle exec`
+  - `bl` aliased to `bundle list`
+  - `bp` aliased to `bundle package`
+  - `bo` aliased to `bundle open`
+  - `bu` aliased to `bundle update`
+  - `bi` aliased to `bundle install --jobs=<cpu core count>` (only for bundler `>= 1.4.0`)
+- adds a wrapper for common gems:
+  - looks for a binstub under `./bin/` and executes it (if present)
+  - calls `bundle exec <gem executable>` otherwise
+
+For a full list of *common gems* being wrapped by default please look at the `bundler.plugin.zsh` file.
+
+## Configuration
+
+Please use the exact name of the executable and not the gem name.
+
+### Add additional gems to be wrapped
+
+Add this before the plugin-list in your `.zshrc`:
+```sh
+BUNDLED_COMMANDS=(rubocop)
+plugins=(... bundler ...)
+```
+This will add the wrapper for the `rubocop` gem (i.e. the executable).
+
+
+### Exclude gems from being wrapped
+
+Add this before the plugin-list in your `.zshrc`:
+```sh
+UNBUNDLED_COMMANDS=(foreman spin)
+plugins=(... bundler ...)
+```
+This will exclude the `foreman` and `spin` gems (i.e. their executable) from being wrapped.
+
+## Excluded gems
+
+These gems should not be called with `bundle exec`. Please see the Issues on GitHub for clarification.
+
+`berks`
+`foreman`
+`mailcatcher`
+`rails`
+`ruby`
+`spin`