Add README for the xcode plugin

plugins/xcode/README.md

+# Xcode
+
+## Description
+
+This plugin provides a few utilities that can help you on your daily use of Xcode and iOS development.
+
+To start using it, add the `xcode` plugin to your `plugins` array:
+
+```zsh
+plugins=(... xcode)
+```
+
+
+## Aliases
+
+| Alias | Description                           | Command                                        |
+|-------|---------------------------------------|------------------------------------------------|
+| xcsel | Change active developer directory     | sudo xcode-select --switch                     |
+| xcp   | Show currently active dev. directory  | xcode-select --print-path                      |
+| xcb   | Build Xcode projects and workspaces   | xcodebuild                                     |
+| xcdd  | Purge all temporary build information | rm -rf ~/Library/Developer/Xcode/DerivedData/* |
+
+
+## Functions
+
+- **`xc`**:
+  Open one of the `.xcworkspace` and `.xcodeproj` files that it can find in the current working directory.
+  Returns 1 if it didn't find any relevant files.
+
+- **`simulator`**:
+  Open the iOS Simulator from your command line, dependant on whichever is the active developer directory for Xcode.