add README with install instructions

README.md

@@ -0,0 +1,13 @@
+# Installation
+
+```tmux
+# install via tpm to get the sysusage_cmd 
+set -g @plugin 'https://gitea.mschirmer.com/m13r/tmux-cpu-mem-monitor'
+
+# ----- Theme ----- #
+CPU_MEM_USAGE="#(#{@sysusage_cmd})"
+
+set-option -g status-right "${CPU_MEM_USAGE}"
+```
+
+![example of how it could look](image.png)