docs(sterni/acme): document mkbqnkeyboard.bqn

Change-Id: Ifc2fd45712bda4d7700b3ba0dea571ee3f7fde0c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/13100
Reviewed-by: sterni <sternenseemann@systemli.org>
Autosubmit: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

users/sterni/acme/README.md

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+# //users/sterni/acme
+
+Utilities and configurations related to the [acme][] text editor
+from [Plan 9][].
+
+## mkbqnkeyboard.bqn
+
+[mkbqnkeyboard.bqn][] is a script that updates a given Plan 9 `/lib/keyboard`
+file to support the familiar [BQN keymap][] via compose sequences. Since
+it uses the GNU Readline [inputrc distributed with BQN][inputrc] as a database
+for the keymap, you can use a [remapped][] version of the layout.
+Once applied, you'll be able to type the Unicode characters used by BQN
+via <kbd>Compose</kbd> followed by <kbd>\\</kbd> and the
+<kbd>mapped ASCII character</kbd>. For details on the file and what button
+is used for <kbd>Compose</kbd>, refer to
+[keyboard(6)][p9f-keyboard] (see also
+[plan9port's keyboard(7)][p9p-keyboard], [9front's keyboard(6)][9front-keyboard] etc.).
+
+TIP: [mkbqnkeyboard.bqn][] has only been tested with [plan9port][],
+so the instructions below may not work with every Plan 9 variant.
+If you have any any information on/trouble with getting it to work on
+a proper Plan 9 (fork), feel free to [let me know][me] or
+[send a patch][submitting-patches].
+
+The process for updating `/lib/keyboard` with [mkbqnkeyboard.bqn][] is a follows:
+
+1. Prerequisites:
+
+   - [CBQN][] (other BQN implementations are untested)
+   - A local checkout of [mlochbaum/BQN][],
+     `aecb56a323aa` is the latest tested revision.
+
+2. Run
+
+       ./mkbqnkeyboard.bqn -i /path/to/mlochbaum/BQN/editors/inputrc /path/to/lib/keyboard
+
+   If you omit `-i`, the modified `keyboard` file will be printed to stdout instead
+   of written to the file. If you add `-s`, the result will be sorted by (resulting) codepoint.
+
+3. If you're using **plan9port**, you'll need to
+
+   1. Apply [latin1-increase-compose-capacity.patch][] since the default compose
+      sequence lookup table is too small to hold all the mappings BQN adds.
+   2. Recompile plan9port.
+
+   Other Plan 9 variants may also require extra steps.
+
+4. The compose sequences should now work in the [acme][] and [sam][] text editors
+   as well as all other Plan 9 programs.
+
+[acme]: https://9p.io/sys/doc/acme/acme.pdf
+[sam]: https://9p.io/sys/doc/sam/sam.pdf
+[Plan 9]: https://p9f.org/about.html
+[BQN keymap]: https://mlochbaum.github.io/BQN/keymap.html
+[me]: https://grep.tvl.fyi/search/?q=%20path%3Aops%2Fusers%2Fdefault.nix%20name%20%3D%20%22sterni%22%3B&fold_case=auto&regex=false&context=true
+[mkbqnkeyboard.bqn]: ./mkbqnkeyboard.bqn
+[inputrc]: https://github.com/mlochbaum/BQN/blob/master/editors/inputrc
+[remapped]: https://mlochbaum.github.io/BQN/editors/index.html#alternate-layouts
+[p9f-keyboard]: https://p9f.org/magic/man2html/6/keyboard
+[p9p-keyboard]: https://9fans.github.io/plan9port/man/man7/keyboard.html
+[9front-keyboard]: http://man.9front.org/6/keyboard
+[plan9port]: https://9fans.github.io/plan9port/
+[submitting-patches]: https://code.tvl.fyi/about/docs/REVIEWS.md
+[mlochbaum/BQN]: https://github.com/mlochbaum/BQN
+[CBQN]: https://github.com/dzaima/cbqn
+[latin1-increase-compose-capacity.patch]: ./plan9port/latin1-increase-compose-capacity.patch