add bugfix in CHANGELOG and some notes in pg README

Closes #192

CHANGELOG.md

@@ -15,6 +15,7 @@ will still be backwards compatible if only `url` is specified.
 - Fix net plugin on darwin
 - [#84](https://github.com/influxdb/telegraf/issues/84): Fix docker plugin on CentOS. Thanks @neezgee!
 - [#189](https://github.com/influxdb/telegraf/pull/189): Fix mem_used_perc. Thanks @mced!
+- [#192](https://github.com/influxdb/telegraf/issues/192): Increase compatibility of postgresql plugin. Now supports versions 8.1+
 
 ## v0.1.8 [2015-09-04]
 

plugins/postgresql/README.md

@@ -0,0 +1,30 @@
+# PostgreSQL plugin
+
+This postgresql plugin provides metrics for your postgres database. It currently works with postgres versions 8.1+. It uses data from the built in _pg_stat_database_ view. The metrics recorded depend on your version of postgres. See table:
+```
+pg version      9.2+   9.1   8.3-9.0   8.1-8.2   7.4-8.0(unsupported)
+---             ---    ---   -------   -------   -------
+datid*           x      x       x         x
+datname*         x      x       x         x
+numbackends      x      x       x         x         x
+xact_commit      x      x       x         x         x
+xact_rollback    x      x       x         x         x
+blks_read        x      x       x         x         x
+blks_hit         x      x       x         x         x
+tup_returned     x      x       x
+tup_fetched      x      x       x
+tup_inserted     x      x       x
+tup_updated      x      x       x
+tup_deleted      x      x       x
+conflicts        x      x
+temp_files       x
+temp_bytes       x
+deadlocks        x
+blk_read_time    x
+blk_write_time   x
+stats_reset*     x      x
+```
+
+_* value ignored and therefore not recorded._
+
+More information about the meaning of these metrics can be found in the [PostgreSQL Documentation](http://www.postgresql.org/docs/9.2/static/monitoring-stats.html#PG-STAT-DATABASE-VIEW)