updated README.md advising against using Cygwin

README.md

@@ -10,7 +10,8 @@ Installation
 
 Requirements:
 
-  - tmux **`>= 2.1`** running inside Linux, Mac, OpenBSD or Cygwin
+  - tmux **`>= 2.1`** running inside Linux, Mac, OpenBSD, Cygwin or WSL (Bash on
+    Ubuntu on Windows)
   - outside of tmux, `$TERM` must be set to `xterm-256color`
 
 To install, run the following from your terminal: (you may want to backup your
@@ -277,6 +278,11 @@ Once installed, `reattach-to-usernamespace` will be automatically detected.
 
 ### Using the configuration under Cygwin within Mintty
 
+**I don't recommend running this configuration with Cygwin anymore. Forking
+under Cygwin is extremely slow and this configuration issues a lot of
+`run-shell` commands under the hood. As such, you will experience high CPU
+usage. As an alternative consider using [Mintty terminal for WSL][wsltty].**
+
 ![cygwin](https://cloud.githubusercontent.com/assets/553208/19741789/67a3f3d8-9bc2-11e6-9ecc-499fc0228ee6.png)
 
 It is possible to use this configuration under Cygwin within Mintty, however