Added a section on proxying to the contributor docs

[kyle-sammons]

Summary

Adds a section on how to setup a proxy to a KalDB query node to make life easier when developing on the frontend.

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

The following point can be removed after setting up CI (such as Travis) with coverage reports (such as Codecov)

• I've written tests to cover the new code and functionality included in this PR.

The following point can be removed after setting up a CLA reporting tool such as cla-assistant.io

• I've read, agree to, and signed the Contributor License Agreement (CLA).

[github-actions]

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