Add Emacs switching guidance.

Mimiced from the Vim guidance.

Because there are now two of them, make an Editors  section.

book/src/SUMMARY.md

@@ -7,7 +7,9 @@
   - [Keymap](./keymap.md)
   - [Commands](./commands.md)
   - [Language support](./lang-support.md)
-- [Migrating from Vim](./from-vim.md)
+- [Migrating from other editors]
+  - [Vim](./from-vim.md)
+  - [Emacs](./from-emacs.md)
 - [Configuration](./configuration.md)
   - [Themes](./themes.md)
   - [Key remapping](./remapping.md)

book/src/from-emacs.md

@@ -0,0 +1,17 @@
+# Migrating from Emacs
+
+Helix is a modal editor stronly inspired by Vim and Kakoune. Users of doom Emacs
+will find it familiar. Helix follows the `selection → action` model. Where many Emacs
+commands can work with the item under point Helix will require a selection first.
+A cursor in Helix is simply a single width selection.
+
+Helix has buffers too. <space>-b will let you shift between them.
+
+Commands can be prefixed with numbers to change their behavior in much the same way
+as C-u N <command> does in Emacs.
+
+Registers in Helix work much like registers do in Emacs as well. Except there is
+currently no mark saving and moving. Check out the jumplist for an alternative.
+
+Many of the same behaviors from rectangle mark mode can be reproduced using multiple
+cursors.