GitHub PR: Added documentation to the metainfo_metrics example.

Original pull request on GitHub: https://github.com/nomad-coe/nomad/pull/55

examples/metainfo/metainfo_metrics.py

+# This example reads all Python define metainfo and computes
+# basic metrics on how many sections and properties are defined
+# per package
+
 from nomad.metainfo import Section, Property, Quantity, Package
 from nomad.cli.dev import _all_metainfo_packages