added custom request handler page

Custom request handler.md

@@ -0,0 +1,79 @@
+Trilium provides a mechanism for [[scripts]] to open a public REST endpoint. This opens a way for various integrations with other services - a simple example would be creating new note from Slack by issuing a slash command (e.g. `/trilium buy milk`).
+
+## Create note from outside Trilium
+Let's take a look at an example. The goal is to provide a REST endpoint to which we can send title and content and Trilium will create a note.
+
+We'll start with creating a JavaScript backend [[code note|code notes]] containing:
+
+```javascript
+const {req, res} = api;
+const {secret, title, content} = req.body;
+
+if (req.method == 'POST' && secret === 'secret-password') {
+    const targetNoteId = await api.currentNote.getRelationValue('targetNote');
+    
+    await api.createNoteAndRefresh(targetNoteId, title, content);
+    res.send(201);
+}
+else {
+    res.send(400);
+}
+```
+
+This script note has also following two attributes:
+
+* label `customRequestHandler` with value `create-note`
+* relation `targetNote` pointing to a note where new notes should be saved
+
+### Explanation
+
+Let's test this by using a HTTP client to send a request:
+
+```http request
+POST http://my.trilium.org/custom/create-note
+Content-Type: application/json
+
+{
+  "secret": "secret-password",
+  "title": "hello",
+  "content": "world"
+}
+```
+
+Notice the `/custom` part in the request path - Trilium considers any request with this prefix as "custom" and tries to find a matching handler by looking at all notes which have `customRequestHandler` [[label|attributes]]. Value of this label then contains a regular expression which will match the request path (in our case trivial regex "create-note").
+
+Trilium will then find our code note created above and execute it. `api.req`, `api.res` are set to [request](https://expressjs.com/en/api.html#req) and [response](https://expressjs.com/en/api.html#res) objects from which we can get details of the request and also respond.
+
+In the code note we check the request method and then use trivial authentication - keep in mind that these endpoints are by default totally unauthenticated and you need to take care of this yourself.
+
+Once we pass these checks we will just create the desired note using [[Script API]].
+
+## Custom resource provider
+
+Another common use case is that you want to just expose a file note - in such case you create label `customResourceProvider` (value is again path regex).
+
+## Advanced concepts
+
+`api.req` and `api.res` are Express.js objects - you can always look into its [documentation](https://expressjs.com/en/api.html) for details.
+
+### Parameters
+
+REST request paths often contain parameters in the URL, e.g.:
+
+```
+http://my.trilium.org/custom/notes/123
+```
+
+The last part is dynamic so the matching of the URL must also be dynamic - for this reason the matching is done with regular expressions. Following `customRequestHandler` value would match it:
+
+```jsregexp
+notes/([0-9]+)
+```
+
+Additionally this also defines a matching group with the use of parenthesis which then makes it easier to extract the value. The matched groups are available in `api.pathParams`:
+
+```javascript
+const noteId = api.pathParams[0];
+```
+
+Often you also need query params (as in e.g. `http://my.trilium.org/custom/notes?noteId=123`), you can get those with standard express `req.query.noteId`.

_Sidebar.md

@@ -33,6 +33,7 @@
   * [[Code notes]]
     * [[Scripts]]
     * [[Script API]]
+    * [[Custom request handler]]
     * [[Events]]
   * [[Advanced showcases]]
     * [[Day notes]]