release workflow: change default branch to main

Merge pull request #297 from Narice/core

Added note about cache in start/index.md docs

Merge pull request #235 from divnix/release-0.9.0

Prepare 0.9.0 release

workflows#release: set-env is deprecated

https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands

README.md: promote upstreaming

update: small fixes after version bump

flake: update lock file