Add "help wanted" instructions to site

## Content

I would like the instructions on "how to open source" to include information on how to use the Code.gov "help wanted" labels so that GH issues can be displayed on the Code.gov site. Instructions for using these labels can be found on the ["code-gov" repo](https://github.com/GSA/code-gov/blob/master/HelpWanted.md).

## Page(s) Affected

https://www.code.mil/how-to-open-source.html

But maybe also https://www.code.mil/frequently-asked-questions.html ?

Start with the instructions at https://www.code.mil/how-to-open-source.html and compare the FAQ page to determine whether both need updating. Read the linked code-gov HelpWanted.md guidance, then update the relevant content so contributors know how to use the labels and the pages accurately explain the Code.gov integration.

Written by the indexing model from the issue text.