v0.21.0

We are ironing out the last few issues with the conda-forge integration! One noteable one: run dependencies don't need ot be taken into account when computing the variants (for example, if you have numpy only in the run dependencies of a noarch package and three versions in the variant configuration, then we only create a single package now). This aligns with the behavior of conda-build.

What's Changed

• refactor: make build string optional in recipe by @baszalmstra in #1020
• fix: CLI docs by @Hofer-Julian in #1036
• fix: used vars with run dependencies by @wolfv in #1037
• chore: prepare release 0.21.0 by @wolfv in #1041

Full Changelog: v0.20.0...v0.21.0