Add readme

Author: ScoreUnder

README.md

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+# CPU usage monitor
+
+## TL;DR
+
+* shows CPU usage percentage
+* needs systemd (kinda)
+* made for i3blocks
+
+# Description
+
+Intended for use on systemd systems. Maybe uncomment line 9 and comment out
+line 10 if you're not using systemd. (At the expense of a ridiculously slim
+chance of a hypothetical symlink attack)
+
+This tool will output the CPU usage percentage to stdout and then exit. The
+first run will produce the output "nan%" but the rest will be fine. Designed
+for i3blocks as a drop-in replacement for the existing script. The existing one
+lingers around for a short moment and shows CPU usage between program start and
+finish, but this one shows CPU usage between invocations instead, which means
+that all you need to do to change the resolution in i3blocks is to change the
+i3blocks "interval" as with any other program.
+
+# How to build
+
+1. install dune (the ocaml build tool)
+2. `dune build`