Add WSL Instructions

[mcdonnnj]

🗣 Description

This PR adds instructions for the Windows Subsystem for Linux (WSL) to setup pyenv for development usage. There is also clarification about prerequisites and what lines to add to your .bashrc for Linux installations.

💭 Motivation and Context

While trying to get INL setup with our development practices on WSL there was a lot of troubleshooting. This should make it easier for a new user to get their development environment in line with our practices more easily.

🧪 Testing

I started with a fresh installation of the Ubuntu app on a Windows 10 installation and followed the directions as listed. After completing the pyenv installation I was able to install Python versions 3.8.1, 3.7.6, 3.6.10, and 2.7.17 (latest releases of the 3.8, 3,7, 3,6, and 2.7 major versions) with no issues. Those versions should cover reasonable development situations, but are not exhaustive.

🚥 Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (causes existing functionality to change)

✅ Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[dav3r]

Thanks for writing this up @mcdonnnj - it should be very helpful for the INL team! Please consider my one small comment. 📝

[jsf9k]

Thanks for taking care of this @mcdonnnj!

I added a commit that removed an errant space and also cleaned up the line breaking. I did that because I think these changes should be backported to cisagov/skeleton-generic, so I'd like the formatting to be as clean as possible. I like for the *.md documents to be easily readable as plain text without rendering in Markdown.