clarify

Author: erikw

README.md

@@ -81,6 +81,7 @@ Do you enjoy hacking tmux and your shell? As less time is becoming available for
 Your profile could look like:
 * Use tmux regularly, configured with tmux-powerline.
 * Experience in writing maintainable, correct and understandable bash scripts, or willing to become proficient. Some prior knowledge would be needed.
+  * Understanding of portability of shell scripting (POSIX) e.g. that some command line swithces to common tools might differ on Linux to BSD or macOS.
 * When following the code paths from [powerline.sh](powerline.sh) to [lib/powerline.sh](lib/powerline.sh) understand how the powerline is rendered.
 * Understand how the segment's different functions are called and expected to behave.
 * Understand how themes and the config file works.