docs: add custom mode description

Author: mishamyrt

docs/custom-mode.md

@@ -0,0 +1,48 @@
+# Backlight custom effect
+
+In addition to the preset effects, the keyboard is able to display a custom one. It is static (has no animation) and allows you to configure separate colors for each key.
+
+## Read
+
+To read a custom effect, you have to send a request:
+
+```sh
+0x05 0x89 0xBC 0x00 0x00 0x00
+```
+
+The response will be a sequence of 1050 bytes. The colors for the keys are encoded in these bytes.
+
+The first will be the header, which is always `0x89`. Followed by 6 zeros.
+
+Then comes the effect colors. Keys positions are sampled from byte 7. You can see an example of positions in [template_backlight.go](../layout/template_backlight.go).
+
+Colors are written in a rather strange way: first comes red, after 126 bytes comes green and after another 126 bytes comes blue.
+That is, if you have white color written for the first and second button, then 255 will be written to the bytes under the indexes:
+
+```sh
+# First
+7, 133, 259
+# Second
+8, 134, 260
+```
+
+## Write
+
+To record the effect you need to send a sequence of bytes in the same format but with a different header. Cut off the first 7 bytes and add the command:
+
+```sh
+0x06 0x09 0xBC 0x00 0x40 0x00 0x00 0x00 # command
+0xFF 0xFF 0x00 0x00 0x00 0x00 0x00 0x00 # payload
+...
+```
+
+## Pages
+
+It seems that the keyboard can store several pages of custom effects, but I haven't figured out how to activate them yet. There are the following commands to work with them:
+
+```sh
+# Read
+0x05 0x89 0xC0 0x00 0x00 0x00
+# Write
+0x06 0x09 0xC0 0x00 0x40 0x00 0x00 0x00
+```