Add installation steps for tmux 2.3 and earlier

Author: bruno-

README.md

@@ -88,6 +88,13 @@ be able to use the plugin.
 Optional (but recommended) install `gawk` via your package manager of choice
 for better UTF-8 character support.
 
+### Installation for Tmux 2.3 and earlier
+
+Due to the changes in tmux, the latest version of this plugin doesn't support
+tmux 2.3 and earlier. It is recommended you upgrade to tmux version 2.4 or
+later. If you must continue using older version, please follow
+[these steps for installation](docs/installation_for_tmux_2.3.md).
+
 ### Limitations
 
 This plugin has some known limitations. Please read about it

docs/installation_for_tmux_2.3.md

@@ -0,0 +1,20 @@
+# Installation for Tmux 2.3 and earlier
+
+The installation steps for Tmux 2.3 are based on
+[manual installation](https://github.com/tmux-plugins/tmux-copycat#manual-installation)
+steps, with the addition of using `tmux-23` branch.
+
+Create tmux plugins dir:
+
+    $ mkdir -p ~/.tmux/plugins
+
+Clone the repo:
+
+    $ git clone -b tmux-23 https://github.com/tmux-plugins/tmux-copycat ~/.tmux/plugins/tmux-copycat
+
+Add this line to the bottom of `.tmux.conf`:
+
+    run-shell ~/clone/path/copycat.tmux
+
+Reload TMUX environment with: `$ tmux source-file ~/.tmux.conf`. You should now
+be able to use the plugin.