Update documentation

Author: JanLuca

docs/source/general.rst

@@ -26,7 +26,28 @@ Python package manager pip:
 Usage
 =====
 
-ToDo
+The :obj:`varipeps` module is organized in several submodules corresponding to
+the different features. For a variational optimization the most important parts
+are (a full overview can be found in the :ref:`_api`):
+
+* :obj:`varipeps.peps`: To define iPEPS unit cell and the tensors on each site,
+  the library provides in this submodule the abstractions to define such a unit
+  cell.
+* :obj:`varipeps.expectation`: In this submodule the helper functions to define
+  and calculate common expecation functions on the iPEPS unit
+  cell. Particularly, the function can be used to define the Hamiltonian terms
+  of the model of interest.
+* :obj:`varipeps.mapping`: If not only interactions on the square lattice are of
+  interest but also models on other 2d lattices, in this submodule one can find
+  mappings of other lattices. Also the files there can be a good starting point
+  to implement even more lattices.
+* :obj:`varipeps.optimization`: The submodule providing the optimization
+  algorithm and interface of the library. In almost all cases, one will interact
+  with this part by the main function
+  :obj:`varipeps.optimization.optimize_peps_network`.
+
+All these different modules can be seen in action in the :ref:`_examples`
+section of the documentation where exemplary code is discussed in detail.
 
 Citation
 ========