Add api docs for settings endpoint

docs/development/API/pleroma_api.md

@@ -695,3 +695,42 @@ Emoji reactions work a lot like favourites do. They make it possible to react to
 * Authentication: required
 * Params: none
 * Response: HTTP 200 on success, 500 on error
+
+## `/api/v1/pleroma/settings/:app`
+### Gets settings for some application
+* Method `GET`
+* Authentication: `read:accounts`
+
+* Response: JSON. The settings for that application, or empty object if there is none.
+* Example response:
+```json
+{
+  "some key": "some value"
+}
+```
+
+### Updates settings for some application
+* Method `PATCH`
+* Authentication: `write:accounts`
+* Request body: JSON object. The object will be merged recursively with old settings. If some field is set to null, it is removed.
+* Example request:
+```json
+{
+  "some key": "some value",
+  "key to remove": null,
+  "nested field": {
+    "some key": "some value",
+    "key to remove": null
+  }
+}
+```
+* Response: JSON. Updated (merged) settings for that application.
+* Example response:
+```json
+{
+  "some key": "some value",
+  "nested field": {
+    "some key": "some value",
+  }
+}
+```