Add quick usage guide for Grapnel

[skip ci]

Grapnel/README.md

@@ -0,0 +1,41 @@
+# Grapnel hooks for Limnoria
+
+**Grapnel** allows announcing Slack-style webhooks to IRC channels, via Limnoria's HTTP server. Currently, only plain text payloads are supported.
+
+## Usage
+
+1) Configure the public base URL of your web server: `config plugins.grapnel.baseurl https://address.of.your.bot`
+
+2) Create a webhook: `grapnel add [network] [channel]`
+    - `network` and `channel` default to the current network and channel, if you run this command in a channel.
+
+3) Save the URL generated and plug it into whatever app you want to announce events from.
+    - Replace `your-cool-app-name` with the name of the sending app; it will be reflected in the output by default
+    - **Make sure to keep this link private, as it includes the token!**
+
+4) Test the webhook: e.g.
+
+```bash
+$ curl -X POST -d '{"text": "hello world"}' -H 'Content-type: application/json' 'https://address.of.your.bot/grapnel/1?token=xxxxxx&sender=myapp'
+```
+
+### Command index
+
+- **`grapnel add [<network>] [<channel>]`** -- Creates a new Slack-compatible webhook endpoint for a given network + channel. `network` and `channel` default to the current network and channel if not specified.
+- **`grapnel get <webhook ID>`** -- Returns metadata and the URL for the webhook ID.
+- **`grapnel listhooks [<network>] [<channel>]`** -- Lists webhooks set on the network + channel pair. `network` and `channel` default to the current network and channel if not specified.
+- **`grapnel remove <webhook ID>`** -- Removes the given webhook ID.
+- **`grapnel resettoken <webhook ID>`** -- Regenerates the token for the given webhook ID.
+
+### Output template
+
+The `plugins.grapnel.format` option can be adjusted to change the output format. It supports these fields:
+
+- `$name` (the sender name from the `sender=` part of the URL)
+- `$text`
+- Limnoria's standard substitutions:
+    - `$randomInt`
+    - `$botnick`
+    - `$today`
+    - `$now`
+    - `$randomDate`

README.md

@@ -34,6 +34,9 @@ Most of these plugins also have their own READMEs in their folders; you can usua
 ### AQI
 - Retrieves [air quality index](https://en.wikipedia.org/wiki/Air_quality_index) info from the [World Air Quality Index project](https://aqicn.org).
 
+### [Grapnel](Grapnel/README.md)
+- Announce Slack-compatible webhooks to IRC
+
 ### LastFM
 - LastFM plugin, forked from [krf/supybot-lastfm](https://github.com/krf/supybot-lastfm).