Add docstring for structure.alphabet subpackage

src/biotite/structure/alphabet/__init__.py

@@ -4,6 +4,18 @@
 
 """
 A subpackage for converting structures to structural alphabet sequences.
+
+Structural alphabets represent the local geometry of each residue in a structure as
+symbol in a sequence.
+This allows using sequence-based functionality from :mod:`biotite.sequence` on
+structural data.
+
+For each supported structural alphabet, this subpackage provides a conversion function
+that converts each chain of a given structure into a :class:`Sequence` object from the
+respective structural alphabet.
+
+Note that the structural alphabets use lower-case letters as symbols, in order to
+distinguish them better from the nucleotide and amino acid alphabets.
 """
 
 __name__ = "biotite.structure.alphabet"