README: clarify deprecation and point to modules

[mvdan]

See golang/go#38158.

[mvdan]

FYI @myitcv @ALTree; had to create a PR since the master branch is locked against direct pushes.

[mvdan]

Oh, the docs site is also in the master branch. I'll fix that here too.

[myitcv]

LGTM with one suggestion.

[ALTree]

LGTM but I think the Feedback and Contributing sections should be
deleted from the README. Installing is still relevant since it's
possible to download an archived repository, but it won't be possible
to give feedback or contribute.

[mvdan]

You both raise good points :) I'll update the commit.

Also, of course CI is required to merge, and it's broken... we'll have to wait for @andybons to grant one of us admin access to be able to push to master.

[ALTree]

Indeed, the build is broken, so I've just disabled the hard requirement on the travis-ci check for merges. Now it should allow the merge even if the travis-ci build failed.

[mvdan]

I ended up with **NOTE:** instead of ** NOTE: **, because only the former would actually render as bold. I love markdown sometimes.

[mvdan]

I think the Feedback and Contributing sections should be deleted from the README

I've also deleted both of those sections. Merging - thanks both for the reviews!

[mvdan]

It looks like actually updating the generated gh-pages branch is a manual process, last done in 2018. I'll try to figure out how to do it and push there.

[mvdan]

Alright, done: 7afd7b6

All that I did boiled down to:

cd website
yarn
yarn build
GIT_USER=mvdan yarn publish-gh-pages

There are other changes applied to the generated pages aside from this PR, which I attribute to the long time it's been since anyone ran the tool. They all seem minor, though.