fix meta payload attribute in README

Payload is in `content=`, not `value=`.

README: Link to IMPLEMENTATIONS.md

IMPLEMENTATIONS: add vanitydoc

README: Link to RFC.md

IMPLEMENTATIONS: Add SourceHut and Godocs.io

Revert "RFC(3.2): Trailing slash convention, not requirement"

This reverts commit 70531cf9ac2bba70a0b008ec7dc544c0a966f311.

As per discussion on mailing list[1], this is likely to cause issues.

[1] https://lists.sr.ht/~ancarda/vcs-autodiscovery/%3CCFA7DXTFDWXD.1YR4U9KQT6YJ0%40nitro%3E#%3CCFVPD97TLC7Y.2PC9NK6O9SITN@nitro%3E

RFC: Fix BCP 14 usage (MUST not -> MUST NOT)

In 3.2 and 4.1, the text "MUST not" was suppose to be "MUST NOT", as per
BCP 14.

RFC(3.2): Trailing slash convention, not requirement

RFC(3.2): Exception regarding root directory

RFC(4): Restructure to have vcs and forge

Following feedback from Adnan Maolood (~adnano), section 4 was heavily
rewritten. Most meta tags starting with "vcs:" have been replaced with
"forge:".

RFC(3.1): Replace branch with ref

Not every version control system has branches, so it has been replaced
with ref, which is more neutral and system-agnostic.

Thanks to Adnan Maolood (~adnano) for pointing this out.

RFC: Replace % with curly braces

% is a reserved character in URLs and so is a poor choice for denoting
variables. It has been replaced with curly braces.

Thanks to Adnan Maolood (~adnano) for pointing this out.

4.3: Fix typo (file -> rawfile)

The code example should have been `rawfile', not `file'.

Initial commit