README.md: Add instructions. 💡

README.md

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+# debounce-mac
+
+This is for those annoying moments when you wished that macOS had
+[`XkbSetBounceKeysDelay`](https://linux.die.net/man/3/xkbsetbouncekeysdelay)
+like Linux does.
+
+This is a bog-simple keyboard event tap that intercepts any
+(identical) keystrokes that happen within a configurable time window.
+Magic.  [See
+here](https://apple.stackexchange.com/questions/246840/debounce-mechanical-keyboard-in-os-x)
+for the genesis story.
+
+Credit where credit is due: i mostly stole this idea and sample code
+off another [StackOverflow
+answer](https://stackoverflow.com/questions/19646108/modify-keydown-output).
+
+## How do?
+
+You should be able to make this work with something like the
+following:
+
+```ShellSession
+$ make
+$ sudo ./debounce
+```
+
+It needs root because it intercepts keystrokes.
+
+### Accessibility / permissions
+
+You might get an error something like the following, even when running
+as root.
+
+```ShellSession
+$ sudo ./debounce
+2019-03-03 10:47:02.194 debounce[59588:499245] Initializing an event tap.
+2019-03-03 10:47:02.205 debounce[59588:499245] Unable to create event tap.  Must run as root or add Accessibility privileges to this app.
+2019-03-03 10:47:02.205 debounce[59588:499245] No Event tap in place!  You will need to call listen after tapEvents to get events.
+```
+
+@DanGrayson kindly [alerted me to the
+fact](https://github.com/toothbrush/debounce-mac/issues/4) that on
+modern macOS, you'll need to enable universal accessibility features
+to let this work.  It appears that you'll need to go to System
+Preferences > Privacy > Accessibility, and grant Terminal.app
+permission, not the `debounce` binary as you might expect.