FAQ: Introduce a section describing autocomplete support.

This commit adds a FAQ entry: `How do I use autocompletes in ZT?`.

* Hotkeys used
* Example guide to using autocomplete to pick a matching emoji
* Support in the message body
* Support in the message recipients (stream/topic/PM recipients)

(minor changes for consistency/structure by neiljp)

docs/FAQ.md

@@ -68,6 +68,56 @@ Since we expect the above to be straightforward for most users and it allows the
 However, we are certainly likely to move towards a system to make access of the different servers simpler, which should be made easier through work such as [zulip-terminal#678](https://github.com/zulip/zulip-terminal/issues/678).
 In the longer term we may move to multiple servers per session, which is tracked in [zulip-terminal#961](https://github.com/zulip/zulip-terminal/issues/961).
 
+## How do I use autocompletes in ZT?
+
+Composing messages could be a tiring task when you have to type all things by yourself. Autocompletes come to the rescue here! ZT currently supports some basic autocompletes, for different compose boxes.
+
+### Hotkeys triggering autocomplete
+
+We use `ctrl` + **`f`** and `ctrl` + **`r`** for cycling through autocompletes (**forward** & **reverse** respectively).
+
+**NOTE:** We don't use `tab`/`shift tab` (although it is widely used elsewhere) for cycling through matches. However, recall that we do use `tab` to cycle through recipient and content boxes. (See [hotkeys for composing a message](https://github.com/zulip/zulip-terminal/blob/main/docs/hotkeys.md#composing-a-message))
+
+### Example: Using autocomplete to add a recognized emoji in your message content
+
+1. Type a prefix designated for an autocomplete (e.g., `:` for autocompleting emojis).
+2. Along with the prefix, type the initial letters of the text (e.g., `air` for `airplane`).
+3. Now hit the hotkey. You'd see suggestions being listed in the footer (a maximum of 10) if there are any.
+4. Cycle between different suggestions as described in above hotkey section. (Notice that a selected suggestion is highlighted)
+5. Cycling past the end of suggestions goes back to the prefix you entered (`:air` for this case).
+
+![selected_footer_autocomplete](https://user-images.githubusercontent.com/55916430/116669526-53cfb880-a9bc-11eb-8073-11b220e6f15a.gif)
+
+### Autocomplete in the message content
+
+As in the example above, a specific prefix is required to indicate which action to perform (what text to insert) via the autocomplete:
+
+|Action|Prefix text(s)|Autocompleted text format|
+| :--- | :---: | :---: |
+|Mention user or user group|`@`|`@*User group*` or `@**User name**`|
+|Mention user|`@**`|`@**User name**`|
+|Mention user group|`@*`|`@*User group*`|
+|Mention user (silently)|`@_`, `@_**`|`@_**User name**`|
+|Link to stream|`#`, `#**`|`#**stream name**`|
+|Insert emoji|`:`|`:emoji_name:`|
+
+**NOTE:** Currently, the above autocompletes ignore a text containing whitespaces if it is specified as a prefix for autocompletion. (e.g., `#test he` wouldn't lead to `#**test here**`) See also [#925](https://github.com/zulip/zulip-terminal/issues/925).
+
+### Autocomplete of message recipients
+
+Since each of the stream (1), topic (2) and private message recipients (3) areas are very specific, no prefix must be manually entered and values provided through autocomplete depend upon the context automatically.
+
+![Stream header](https://user-images.githubusercontent.com/55916430/118403323-8e5b7580-b68b-11eb-9c8a-734c2fe6b774.png)
+
+**NOTE:** The stream box prefix is automatic:
+* `#` (with stream color) if the stream is valid and public,
+* `P` (with stream color) if the stream is valid and private,
+* `✗` if the stream is invalid.
+
+![PM recipients header](https://user-images.githubusercontent.com/55916430/118403345-9d422800-b68b-11eb-9005-6d2af74adab9.png)
+
+**NOTE:** If a PM recipient's name contains comma(s) (`,`), they are currently treated as comma-separated recipients.
+
 ## Unable to render symbols
 
 If some symbols don't render properly on your terminal, it could likely be because of the symbols not being supported on your terminal emulator and/or font.