Cleaned up documentation for initial setup for developer tasks.

[iennae]

What does this change

In the initial setup, I was getting an error following

go get -u  get.porter.sh/porter
package get.porter.sh/porter: no Go files in /Users/sigje/go/src/get.porter.sh/porter

This commit cleans up the documentation around initial setup to match what I think
is going on more closely.

What issue does it fix

Closes #1020

Notes for the reviewer

A couple of issues to think about:

• get.porter.sh url versus GitHub
• meta around what install does and whether more process documentation is needed around setup.

I'm not sure whether there is a get.porter.sh url that would work for cloning directly.

This

fatal: unable to update url base from redirection:
  asked for: https://get.porter.sh/porter/info/refs?service=git-upload-pack
   redirect: https://github.com/deislabs/porter

didn't work and maybe there is a better way to do that or maybe it's just a "remember to update when the organization changes".

Additionally, I did clean up the language in the what does make install do. I'm kind of wondering if there is a need to say something around creating a development environment separate from operating environment.

Say you've installed porter following that process, decide you want to do some development work, if you follow the steps as written you just blew away your installation for the development version. That's probably a separate meta/docs issue, but I wanted to bring it up.

Checklist

• Unit Tests
• Documentation
• Schema (porter.yaml)

[iennae]

set PORTER_HOME and modify how they call from so folks don't have to install and override system installed porter.
Example:

$ export PORTER_HOME=~/.porter
$ ./bin/porter list
NAME                     CREATED      MODIFIED     LAST ACTION   LAST STATUS
porter-wordpress         2019-11-11   2019-11-11   install       success
porter-wordpress-mysql   2019-11-11   2019-11-11   install       success

[vdice]

LGTM!