bundler: add alias for bundle add

Fixes #8892

plugins/bundler/README.md

 # Bundler
 
-- adds completion for basic bundler commands
-- adds short aliases for common bundler commands
+- Adds completion for basic bundler commands
+
+- Adds short aliases for common bundler commands
+  - `ba` aliased to `bundle add`
   - `be` aliased to `bundle exec`.
     It also supports aliases (if `rs` is `rails server`, `be rs` will bundle-exec `rails server`).
   - `bl` aliased to `bundle list`
@@ -10,9 +12,12 @@
   - `bout` aliased to `bundle outdated`
   - `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
+  - `bcn` aliased to `bundle clean`
+  - `bck` aliased to `bundle check`
+
+- Adds a wrapper for common gems:
+  - Looks for a binstub under `./bin/` and executes it (if present)
+  - Calls `bundle exec <gem executable>` otherwise
 
 Common gems wrapped by default (by name of the executable):
 `annotate`, `cap`, `capify`, `cucumber`, `foodcritic`, `guard`, `hanami`, `irb`, `jekyll`, `kitchen`, `knife`, `middleman`, `nanoc`, `pry`, `puma`, `rackup`, `rainbows`, `rake`, `rspec`, `rubocop`, `shotgun`, `sidekiq`, `spec`, `spork`, `spring`, `strainer`, `tailor`, `taps`, `thin`, `thor`, `unicorn` and `unicorn_rails`.
@@ -24,20 +29,24 @@ 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

plugins/bundler/bundler.plugin.zsh

+alias ba="bundle add"
 alias be="bundle exec"
 alias bl="bundle list"
 alias bp="bundle package"