docs: write initial documentation

docs/usage.md

@@ -0,0 +1,62 @@
+# Using Nixlets
+
+## Fetching
+
+You can fetch Nixlets you uploaded somewhere [earlier](./packaging.md) using an URL to the tarball and the sha256 hash:
+
+```nix
+nixlet-lib.fetchNixlet "<URL>" "<sha>"
+```
+
+For the GitLab Package Registry there is a helper function:
+
+```nix
+nixlet-lib.fetchNixletFromGitlab {
+  project = "<org>/<project>"; # eg "TECHNOFAB/nixlets"
+  name = "<nixlet>";
+  version = "<version>";
+  sha256 = "<sha>";
+}
+```
+
+## Metadata
+
+A Nixlet's metadata can easily be accessed:
+
+```nix
+(<some nixlet>).description # version, name, etc.
+```
+
+## Rendering
+
+Rendering a Nixlet will produce a YAML file which contains all the resources.
+
+```nix
+(<some nixlet>).render {
+  inherit system;
+  # values = {};
+  # project = "";
+  # overrides = ({...}: {});
+}
+```
+
+Parameters:
+
+- `system`: needed for Kubenix to work.
+- `values`: values to pass to the Nixlet, gets validated by the Nixlets options
+- `project`: project to use, makes it possible to use the same Nixlet multiple
+    times without conflicts
+- `overrides`: custom module which can override configuration
+
+## Just evaluating
+
+Instead of directly rendering a Nixlet it's also possible to just evaluate it
+and access all kinds of data inside it.
+
+```nix
+(<some nixlet>).eval {
+  # see above
+}
+```
+
+It accepts the same parameters as `render`.