doc: add statement about support for dependencies

[mhdawson]

• In the review of doc: add supported platforms list #11872 we
  pulled out the discussion of supporting dependencies. This
  PR adds back that statement.
• Update README.md to indicatte BUILDING.md contains the list
  of supported platforms. ( I missed this part in the original PR)

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows [commit guidelines][]

Affected core subsystem(s)

doc

[mhdawson]

To start the conversation where it was left of in the last PR @bnoordhuis suggested we not include a statement about dependency support:

I'd leave this out. With c-ares it was broken for years. The numerous V8 patches we float basically mean it's only guaranteed to work with our fork.

Historically, we made no promises except that we'll consider patches.

[thefourtheye]

There is no deps directory in the doc/api directory.

[thefourtheye]

Also, prefer [..][..] format to mention links.

[mhdawson]

@thefourtheye, BUILDING.md is not in in the doc/api directory. Its at the top level with the README.md so the link seems to work ok.

In terms of [..][..], it was originally in that format but it was suggested in the review of the last PR that it be changed to .. as it seems people were not aware the [..][..] format was going to work.

[lpinca]

LGTM with @thefourtheye's nit addressed.

[bnoordhuis]

I'm ambivalent. This has potentially far-ranging implications and "intends to support" is stretching it when we don't do any actual testing. "If it works, great; if not, send pull requests" would be more honest.

[mhdawson]

@rvagg can you chime in here ?

Since we don't test building with shared libraries I'd favor just leaving it out.

[mhdawson]

ping @rvagg. Would like your input to decide if we should add a statement with respect to dependencies or just close this PR.