MVP for HTTP API (#2231)

Co-authored-by: Klaas Tammling <klaas@tammling.hamburg>

docs/MANUAL.md

@@ -63,6 +63,7 @@ _Copyright © Daniel Oaks <daniel@danieloaks.net>, Shivaram Lingamneni <slingamn
     - [Tor](#tor)
     - [I2P](#i2p)
     - [ZNC](#znc)
+    - [API](#api)
     - [External authentication systems](#external-authentication-systems)
     - [DNSBLs and other IP checking systems](#dnsbls-and-other-ip-checking-systems)
 - [Acknowledgements](#acknowledgements)
@@ -1175,6 +1176,10 @@ ZNC 1.6.x (still pretty common in distros that package old versions of IRC softw
 
 Ergo can emulate certain capabilities of the ZNC bouncer for the benefit of clients, in particular the third-party [playback](https://wiki.znc.in/Playback) module. This enables clients with specific support for ZNC to receive selective history playback automatically. To configure this in [Textual](https://www.codeux.com/textual/), go to "Server properties", select "Vendor specific", uncheck "Do not automatically join channels on connect", and check "Only play back messages you missed". Other clients with support are listed on ZNC's wiki page.
 
+## API
+
+Ergo offers an HTTP API that can be used to control Ergo, or to allow other applications to use Ergo as a source of truth for authentication. The API is documented separately; see [API.md](https://github.com/ergochat/ergo/blob/stable/docs/API.md) on the website, or the `API.md` file that was bundled with your release.
+
 ## External authentication systems
 
 Ergo can be configured to call arbitrary scripts to authenticate users; see the `auth-script` section of the config. The API for these scripts is as follows: Ergo will invoke the script with a configurable set of arguments, then send it the authentication data as JSON on the first line (`\n`-terminated) of stdin. The input is a JSON dictionary with the following keys: