Added instructions for installing the pg node module

[spobwoode]

I believe this is necessary to deploy to Google app engine - my deployment didn't work until I did this

Description of what you did:

[codecov]

Codecov Report

Merging #6744 into documentation will increase coverage by 0.85%.
The diff coverage is n/a.

@@                Coverage Diff                @@
##           documentation    #6744      +/-   ##
=================================================
+ Coverage          19.14%   19.99%   +0.85%     
=================================================
  Files                855      858       +3     
  Lines              11933    12095     +162     
  Branches            1898     1965      +67     
=================================================
+ Hits                2284     2418     +134     
+ Misses              8100     8092       -8     
- Partials            1549     1585      +36     

Flag	Coverage Δ
#front	14.67% <ø> (+0.18%)	⬆️
#unit	41.71% <ø> (+1.77%)	⬆️

Impacted Files	Coverage Δ
packages/strapi-plugin-email/admin/src/pluginId.js	0.00% <0.00%> (-100.00%)	⬇️
...n/src/containers/NotificationProvider/constants.js	0.00% <0.00%> (-100.00%)	⬇️
...n/admin/src/containers/LanguageProvider/reducer.js	0.00% <0.00%> (-61.54%)	⬇️
packages/strapi-utils/lib/stringFormatting.js	50.00% <0.00%> (-50.00%)	⬇️
packages/strapi-plugin-graphql/services/utils.js	24.69% <0.00%> (-11.03%)	⬇️
...min/src/containers/NotificationProvider/reducer.js	0.00% <0.00%> (-10.00%)	⬇️
.../admin/src/components/LeftMenuLink/LeftMenuIcon.js	50.00% <0.00%> (-7.15%)	⬇️
...strapi-plugin-upload/config/functions/bootstrap.js	62.06% <0.00%> (-6.69%)	⬇️
...api-admin/admin/src/components/PluginCard/index.js	4.87% <0.00%> (-2.27%)	⬇️
...trapi-admin/admin/src/containers/HomePage/hooks.js	5.00% <0.00%> (-0.89%)	⬇️
... and 159 more

---

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[derrickmehaffy]

LGTM and you are correct

[derrickmehaffy]

Can you please change the branch from Master to Documentation though?

See the following: https://github.com/strapi/strapi/blob/master/CONTRIBUTING.md#documentation

[alexandrebodin]

Hey @spobwoode you changed your PR base branch but your PR now contains so commits it shouldn't. Do you mind updating it to only contain one commit :)

[spobwoode]

@alexandrebodin - Sorry, in trying to clean up this PR I think I might have just made it messier :/ Can you tell me how I can fix this? As you might have guessed, I'm a bit of a n00b

[alexandrebodin]

If you are not too familiar with git I would recommend you create a new branch and make a new PR :) Otherwise you definitely should read about git rebase ;)

[alexandrebodin]

@spobwoode do you mind update your PR ?