Merge branch 'improve-readme' into 'master'

Improved README

See merge request ongresinc/pgio!3

README.md

 # pgio
 
 A Java CLI turned in executable using GraalVM that is able to capture disk IO 
-usage stats per process and to turn this info into a stream of data that can be 
-stored and interpreted (using CSV by default but supporting Prometheus) to 
-analyze and find out which part of postgresql is using the disk most during a 
+usage stats per process (that can be grouped by process type) and total of the system.
+The tool produce a stream of data that can be 
+stored and interpreted (using CSV by default or exporting to Prometheus) to 
+analyze and find out which part of PostgreSQL is using the disk most during a 
 period of time.
 
 ## Stats collected
@@ -146,10 +147,45 @@ pattern from [java.util.regex.Pattern](https://docs.oracle.com/javase/8/docs/api
 }
 ```
 
+For example to use with PostgreSQL we create a template using following `postgresql.json`:
+
+```
+{
+  "archiver": [ ".*archiver.*" ],
+  "wal sender": [ ".*wal sender.*" ],
+  "bgwriter": [ ".*writer process.*" ],
+  "autovacuum": [ ".*autovacuum worker process.*" ],
+  "stats": [ ".*stats collector process.*" ],
+  "wal writer": [ ".*wal writer process.*" ],
+  "checkpoint": [ ".*checkpointer process.*" ],
+  "query": [ "postgres: " ],
+}
+```
+
+```
+bin/pgio -D <postgresql data dir> --advanced --group postgresql.json
+```
+
+The expected output will include following groups:
+
+* archiver
+* wal sender
+* bgwriter
+* autovacuum
+* stats
+* wal writer
+* checkpoint
+* query
+
+The idea is that you can extend those groups to include other relevant 
+tools like wal-e, pgbouncer, particual user's queries.
+Grouping stats by process type reduce the number of stats collected 
+and provide more understandable metrics. 
+
 ## Prometheus service
 
 To start pgio as a prometheus service:
 
 ```
 bin/pgio -D <postgresql data dir> --advanced --prometheus-service --prometheus-bind 0.0.0.0 --prometheus-port 9090
-```
\ No newline at end of file
+```