Added brief introduction to using external packages

[jake-arkinstall]

Partially addresses #27, and related to #5

I'm a new user of this project, and I found it rather difficult to understand how external packages were used. After finding issue #5, and seeing some explanation, as well as with help from @jsharpe on the Bazel/Python slack channel, I believe that I understand the process.

I figured that that I should assist by adding some instructions to README.md I don't know if my approach is the intended approach, so I'm happy to make any changes (or for you to make any changes) to suit the intended use. Either way, I think such an explanation is vital for newcomers to utilize this project.

[jake-arkinstall]

I agree with the above comments and have updated my PR with the requested changes.

I am likewise happy to address any further concerns.

[CLAassistant]

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