Huge improvements for docs (#58)

docs/content/doc/practical-instructions/feature.md

@@ -0,0 +1,33 @@
+---
+date: "2019-02-12:00:00+02:00"
+title: "Add a new api endpoint"
+draft: false
+type: "doc"
+menu:
+  sidebar:
+    parent: "practical instructions"
+---
+
+# Add a new api endpoint/feature
+
+Most of the api endpoints/features of Vikunja are using the [common web handler](https://code.vikunja.io/web).
+This is a library created by Vikunja in an effort to facilitate the creation of REST endpoints.
+
+This works by abstracting the handling of CRUD-Requests, including rights check.
+
+You can learn more about the web handler on [the project's repo](https://code.vikunja.io/web).
+
+### Helper for pagination
+
+Pagination limits can be calculated with a helper function, `getLimitFromPageIndex(pageIndex)` 
+(only available in the `models` package) from any page number.
+It returns the `limit` (max-length) and `offset` parameters needed for SQL-Queries.
+
+You can feed this function directly into xorm's `Limit`-Function like so:
+
+{{< highlight golang >}}
+lists := []List{}
+err := x.Limit(getLimitFromPageIndex(pageIndex)).Find(&lists)
+{{< /highlight >}}
+
+// TODO: Add a full example from start to finish, like a tutorial on how to create a new endpoint?