In docker example both containers should run on the host net (#149)

* In docker example both containers should run on the host net 
* remove unecessary portmapping in docs

Author: schnipseljagd

Diff for: README.md

@@ -13,7 +13,7 @@ This package is available for Docker:
 # Start an example database
 docker run --net=host -it --rm -e POSTGRES_PASSWORD=password postgres
 # Connect to it
-docker run -e DATA_SOURCE_NAME="postgresql://postgres:password@localhost:5432/?sslmode=disable" -p 9187:9187 wrouesnel/postgres_exporter
+docker run --net=host -e DATA_SOURCE_NAME="postgresql://postgres:password@localhost:5432/?sslmode=disable" wrouesnel/postgres_exporter
 ```
 
 ## Building and running