Update source.md to include Casper submodule

Hey everyone, This is my first pull request ever, so I'm sorry if I did something wrong but I think I followed the guidelines correctly.

Here's what I updated:

So I updated the source.md to include instructions for the Casper submodule: renaming origin to upstream, and adding origin for the Casper submodule.

So when I was following the installation instructions for "Install from Source," I noticed that when I ran the command 'grunt master' to update everything, it threw some fatal errors: " 'upstream' does not appear to be a git repository," in reference to the submodule Casper. (I included all the error messages I received at the end of this.)

I thought others might have this problem when trying to update the source and thought I would add the instructions I used to simply rename origin to upstream and add origin for the Casper submodule.

I also added the instructions and command "git remote -v" so people could check to make sure they had entered everything correctly.
I removed the instructions and command "git checkout master && git pull upstream master" because I reasoned (A) that if they had just cloned the repository, it should be updated, and (B) if they run the command 'grunt master' - that essentially does the same thing and updates all of the repositories.

I could be wrong about all of this - so I'm totally open to correction and any advice :)

Thanks for all your work!

David Ruess

Edit: p.s. I forgot to include this. This is the fatal error message I receive if I don't have the Casper submodule repository renamed:

fatal: 'upstream' does not appear to be a git repository
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
fatal: run_command returned non-zero status for content/themes/casper
.
Warning: Command failed:
                        git submodule sync
                        git submodule update

                        if ! git diff --exit-code --quiet --ignore-submodules=untracked; then
                            echo "Working directory is not clean, do you have uncommited changes? Please commit, stash or discard changes to continue."
                            exit 1
                        fi

                        git checkout master
                        git pull upstream master
                        yarn
                        git submodule foreach "
                            git checkout master && git pull upstream master
                        "

Already on 'master'
From github.com:TryGhost/Ghost
 * branch                master     -> FETCH_HEAD
   a1ece81c5..2b20ae8f7  master     -> upstream/master
warning ghost@2.25.8: The engine "cli" appears to be invalid.
warning "@tryghost/members-auth-pages > react-stripe-elements@2.0.3" has unmet peer dependency "react@^15.5.4 || ^16.0.0-0".
warning "@tryghost/members-auth-pages > react-stripe-elements@2.0.3" has unmet peer dependency "react-dom@^15.5.4 || ^16.0.0-0".
warning " > bookshelf@0.14.2" has incorrect peer dependency "knex@>=0.6.10 <0.15.0".
Switched to branch 'master'
fatal: 'upstream' does not appear to be a git repository
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
fatal: run_command returned non-zero status for content/themes/casper
.
 Use --force to continue.

Aborted due to warnings.

TryGhost / docs

Update source.md to include Casper submodule #121