Added hint to check that Postgres is installed
Added hint to check that Postgres is installed

The error shown if Postgres is missing is confusing—for me it was along the lines of 

```Installing pg 0.18.2 with native extensions

Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    /Users/user/.rbenv/versions/2.2.2/bin/ruby -r ./siteconf20150907-78059-f9x8cq.rb extconf.rb
checking for pg_config... no
No pg_config... trying anyway. If building fails, please try again with
 --with-pg-config=/path/to/pg_config
checking for libpq-fe.h... no
Can't find the 'libpq-fe.h header
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.
```
To save the next person some time, I suggest we add the "check" to the docs.
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