Add descriptions for all the metrics that postgres_exporter could export

[DarthVagen]

As a user I have following questions when reading the docs:

1. What metrics will postgres_exporter export.
2. What does the metric mean.
   I cannot find the answers in docs, but I found a few detailes in source code, which is rather implicit. So I suggest to add descriptions for all the metrics that postgres_exporter could export into docs, which will help users a lot.