Document installation via ZSH frameworks

Add a section to README.md that details installing it with the three
most commonly used ZSH frameworks.

Signed-off-by: Joe Block <[email protected]>

Author: unixorn

README.md

@@ -1,2 +1,21 @@
 # iTerm2-shell-integration
 Shell integration and utilities for iTerm2
+
+## ZSH framework usage
+
+The repository contains `iTerm2-shell-integration.plugin.zsh` to make it easier to use with ZSH frameworks like zgenom, antigen and oh-my-zsh.
+
+### [Zgenom](https://github.com/jandamm/zgenom)
+
+Add `zgenom load gnachman/iTerm2-shell-integration` to your `.zshrc` with your other load commands, then run `zgenom save` to update its `init.zsh` file. The integration will be automatically loaded the next time you start a ZSH session.
+
+### [Antigen](https://github.com/zsh-users/antigen)
+
+Add `antigen bundle gnachman/iTerm2-shell-integration@main` to your `.zshrc`
+
+☝  **Note** that until <https://github.com/zsh-users/antigen/issues/717> gets fixed in Antigen, it only automatically recognizes plugins on the `master` branch, so you need to explicitly specify `@main` here.
+
+### [Oh-My-Zsh](http://ohmyz.sh/)
+
+1. `git clone --depth 1 https://github.com/gnachman/iTerm2-shell-integration.git ${ZSH_CUSTOM:-~/.oh-my-zsh/custom}/plugins/fzf-zsh-plugin`
+2. Add **iterm2-shell-integration** to your plugin list - edit `~.zshrc` and change `plugins=(...)` to `plugins=(... iterm2-shell-integration)`