Spack user survey 2020 blog post

[tgamblin]

This adds a writeup of our 2020 user survey results. It's not done yet, but all the sections and figures are there.

Figures are from analysis at https://github.com/spack/spack-user-survey.

• [x] Write intro
• [x] add section headers
• [x] add figures
• [x] writeups for remaining sections

[hauten]

OK @tgamblin I'm done making comments