add custom keyboard layout

previously globally defined in /usr/share/X11/xkb/symbols/custom
ref: swaywm/sway#3999

.xkb/symbols/custom

@@ -0,0 +1,9 @@
+default partial alphanumeric_keys
+xkb_symbols "basic" {
+    include "ch(basic)"
+    override key <AD11> { [ bracketleft, braceleft ] };
+    override key <AD12> { [ bracketright, braceright ] };
+    override key <AC10> { [ eacute, egrave ] };
+    override key <AC11> { [ agrave, dead_diaeresis ] };
+    override key <BKSL> { [ dollar, exclam, sterling ] };
+};

README.md

@@ -164,8 +164,9 @@ Supports [SyncTeX] in both directions.
 [urxvt]: http://software.schmorp.de/pkg/rxvt-unicode.html
 
 * [`.Xresources`](.Xresources), picked up by [urxvt] and [sxiv]
-* [`.profile`](.profile)
+* [`.profile`](.profile), environment variables
 * [`.config/user-dirs.dirs`](`.config/user-dirs.dirs`), XDG default directories
+* [`.xkb/symbols/custom`](.xkb/symbols/custom), custom keyboard layout
 * [`.latexmkrc`](.latexmkrc)
 * [`.mozilla/firefox/profiles.ini`](.mozilla/firefox/profiles.ini)
 * [`.thunderbird/profiles.ini`](.mozilla/firefox/profiles.ini)