fix: tuareg: use #'; don't mark (tuareg-opam-update-env) as obsolete & docs: Update README.md

[erikmd]

This makes it possible to:

• keep the "C-c C-w" key-binding within tuareg
• don't trigger a fatal warning at byte-comp because of that
• see opam-switch-mode as a recommended, albeit optional dependency

[erikmd]

FYI I took the opportunity to document opam-switch-mode in the README (there is a dedicated section for other modes):

https://github.com/ocaml/tuareg/blob/ac5428e651f994b3f1a171e2b08ee9f8e835c97b/README.md#opam-switch-mode

( Sorry, I could have done this in my earlier PR #307 )

[monnier]

FYI I took the opportunity to document opam-switch-mode in the README (there is a dedicated section for other modes):

Merci Erik, installed,

    Stefan

[erikmd]

@monnier

Merci Erik,

👍 de même, merci pour le cherry-pick.

Now closing the PR.