Fix readme

[lukesonnet]

This is a simple PR that fixes a few things about the Readme files:

• actually sets a height unit that (I think) looks better and runs with fa()
• re-knit the .md file from .Rmd because links at the bottom were broken (https://declaredesign.org/library/articles/how_to_write_and_contribute_designs.html should be https://declaredesign.org/r/designlibrary/articles/How_to_Write_and_Contribute_Designs.html but it was only changed in the Rmd)
• Add the .Rmd to .Rbuildignore