README is outdated

[kosciCZ]

Hey!

I recently attempted contribution (#1212), and had a bit of a tough time getting up and running, because the README is partially outdated. Couple examples:

• the make install-tools target no longer exists in the Makefile
• the make docs-preview target no longer exists in the Makefile
• this might be my inexperience in the golang ecosystem, but it took me a while to figure out I should use asdf to get the right go version installed based on .tool-versions file. Not sure if it is worth mentioning in the README, or if this is just a standard that I'm unfamiliar with.

I'd open up another PR that fixes this, but I'm not really sure on what the correct solution is, so I'm opening an issue instead.

[zregvart]

I think the references to targets not present in the Makefile should be removed. We don't mandate the use of asdf, though some of us use it and we do make sure it is up to date with the version in the go.mod. In general using any recent version of golang should be okay, as it will download the appropriate version as needed.