docs: document importing nixlets

docs/importing.md

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+# Importing Nixlets
+
+Nixlets can now define dependency Nixlets and handle them similarly to how nested
+Helm Charts work.
+
+## Importing
+
+To define a dependency Nixlet, give it a name and pass the Nixlet as a value:
+
+```nix title="default.nix of Nixlet"
+  nixlet.dependencies."postgres" = <any nixlet>;
+```
+
+`<any nixlet>` here could be stuff like `nixlet-lib.fetchNixletFromGitlab {...}`,
+`nixlet-lib.fetchNixlet <url> <sha>`, etc.
+
+## Defining Values
+
+You can pre-define values for dependency Nixlets like this:
+
+```nix title="values.nix of Nixlet"
+  options = {
+    # options for the current Nixlet
+  };
+  # overwriting the default of dependency Nixlets (the user can still overwrite this)
+  config."postgres".replicaCount = 10;
+```
+