Docs: label printing webhook

docs/label-printing.md

@@ -0,0 +1,40 @@
+Label printing
+====
+
+To enable label printing, set `FEATURE_FLAG_LABELPRINTER` to `true`in your `config.php`. You also need to provide a webhook target that is responsible for printing.
+
+Why webhook?
+---
+
+Label printers come in all shapes and forms, and your particular one is probably not the one used by the author of this feature. Also, grocy may does not have a
+direct connection to a local label printer (e.g. grocy is hosted in a cloud vps). Thus, a lightweight implementation is provided by grocy: whenever something
+should print, a POST request to a configured URL is made. The target then is responsible for label printing.
+
+Reference implementation
+---
+
+The webhook was developed and tested against a Brother QL-600 label printer, using endless 62mm label paper. The webhook provider implementation was
+implemented into [a fork of brother_ql_web](https://github.com/mistressofjellyfish/brother_ql_web).
+
+Webhook request
+---
+
+Requests can be configured to be sent server-side (that is, from the machine hosting grocy through GuzzleHttp) or by an AJAX request directly from the browser.
+The latter is neccesary for situations where the grocy hosting machine cannot reach your label printer, however server-side requests are a bit faster and
+tend to be more stable.
+
+Both methods fire this request upon printing:
+
+```
+POST /your/printing/api/endpoint HTTP/1.1
+
+product=<productname>&grocycode=grocy:x:xxx&duedate=DD:%2021-06-09&...
+
+```
+
+If specified, the request body may also be JSON encoded, however the fields stay the same.
+
+Additional POST parameters (like the font to use) may be supplied in `config.php`. Keep in mind that these config values will be distributed to all clients on all requests
+if the webhook is configured to run client-side.
+
+The webhook receiver is required to layout and print the resulting label.