doc: Update README for --enable-dev-mode

[michaelfolkson]

Since #1079 the --enable-dev-mode flag turns on all experimental modules so update README to reflect this.

[real-or-random]

Hm, dev-mode was "hidden" on purpose. I don't think we should encourage people to enable experimental modules, at least not without calling them "experimental" explicitly. Maybe adding the text to the README was actually an oversight in the PR that added examples.

[real-or-random]

In fact, after #1091 we won't have experimental modules, so #1091 should remove the sentence "For experimental modules, you will also need --enable-experimental as well as a flag for each individual module, e.g. --enable-module-ecdh." This was overlooked because the PR was opened before this text existed.

This will resolve the problem for that part in the README, and I feel the README is the wrong place to mention the option. There's also reason why ./configure --help doesn't show it. I think it would be okay to document the option in some development docs (but we don't have development docs, and adding them just to point this out seems overkill.)

[michaelfolkson]

Ok no worries, closing.