docs(faq): how to compose messages in another editer

Author: mek-yt

docs/FAQ.md

@@ -321,6 +321,31 @@ We think of Zulip Terminal as a keyboard-centric client. Consequently, while fun
 
 If any of the hotkeys don't work for you, feel free to open an issue or discuss it on [#zulip-terminal](https://chat.zulip.org/#narrow/stream/206-zulip-terminal).
 
+## Can I compose messages in another editor ?
+
+In the main branch of zulip-terminal, you can now use an external editor to 
+compose your message using `ctrl o` shortcut. If `ZULIP_EDITOR_COMMAND` or 
+`EDITOR` environment variable is set, this command or program would be used 
+to open the message by appending a temporary file filepath of the current message.
+
+It will work directly for most terminal editors with only the program name `vim`, 
+`nano`, `helix`, `kakoune`, `nvim`...
+
+It can also be used for desktop editor with some constraint which needs to be 
+address using `ZULIP_EDITOR_COMMAND` custom command. The program must not fork 
+or detach from the running terminal and should open in a new window, some 
+examples:
+
+- [lapce](https://github.com/lapce/lapce) with `lapce -n -w`
+- [sublime-text](https://www.sublimetext.com/) with `subl -n -w`
+- [marker](https://github.com/fabiocolacio/Marker) with `marker`
+- [vim](https://github.com/vim/vim) with `vim -g -f` or `gvim -f`
+- [vscode](https://github.com/microsoft/vscode) with `code -n -w`
+
+When the external editor process ends (closing the window or quitting terminal 
+editor), the composing box will be updated with the new message content from 
+the temporary file.
+
 ## Zulip-term crashed!
 
 We hope this doesn't happen, but would love to hear about this in order to fix it, since the application should be increasingly stable! Please let us know the problem, and if you're able to duplicate the issue, on the github issue-tracker or at [#zulip-terminal](https://chat.zulip.org/#narrow/stream/206-zulip-terminal).