Setup/install: List common problems in running the pipeline

[liquidsteves]

Back in October, me and Rapi were both having problems with our ssh credentials, where we ended up typing our github username/password repeatedly every time we ran a serge cycle.

Anything else?

[liquidsteves]

1. Getting a Serge directory does not exist during sync and pull
   SERGE_TS=${TMS_DATA}/sege/ts isn't replacing ${TMS_DATA} for some people
   Perhaps it doesn't work in every environment. We can explicitly write out those paths instead if that's the case.

2. Directory Permissions issue. It may look like the following:
   Deleting directory '/var/serge/vcs/'
   Creating directory '/var/serge/vcs/'
   fatal: Could not get current working directory: No such file or directory
   Exception occurred while processing configuration file:
   Exit code: -128; last error: No such file or directory
   The fix for this is to run your command with sudo. sudo serge pull sampleconfig.serge --initialize which will give your environment the right privileges to create/destroy directories.