iterm2: add README (#7230)

plugins/iterm2/README.md

+# iTerm2 plugin
+
+This plugin adds a few functions that are useful when using [iTerm2](https://www.iterm2.com/).
+
+To use it, add _iterm2_ to the plugins array of your zshrc file:
+```
+plugins=(... iterm2)
+```
+
+## Plugin commands
+
+* `_iterm2_command <iterm2-command>`
+  executes an arbitrary iTerm2 command via an escape code sequence.
+  See https://iterm2.com/documentation-escape-codes.html for all supported commands.
+
+* `iterm2_profile <profile-name>`
+  changes the current terminal window's profile (colors, fonts, settings, etc).
+  `profile-name` is the name of another iTerm2 profile. The profile name can contain spaces.
+
+* `iterm2_tab_color <red> <green> <blue>`
+  changes the color of iTerm2's currently active tab.
+  `red`/`green`/`blue` are on the range 0-255.
+
+* `iterm2_tab_color_reset`
+  resets the color of iTerm2's current tab back to default.
+
+## Contributors
+
+- [Aviv Rosenberg](https://github.com/avivrosenberg)