Add link to requirement strings explanation in the manifest.md doc

[vrinek]

No description provided.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @huonw (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see CONTRIBUTING.md for more information.

[alexcrichton]

@bors: r+ 96b2e3a

Thanks!

[bors]

⌛ Testing commit 96b2e3a with merge 9915971...

[bors]

💔 Test failed - cargo-win-32

[alexcrichton]

@bors: retry

[bors]

⌛ Testing commit 96b2e3a with merge 4a8609d...

[bors]

💔 Test failed - cargo-win-32

[alexcrichton]

@bors: retry

[bors]

⌛ Testing commit 96b2e3a with merge 395b7f4...

[bors]

☀️ Test successful - cargo-linux-32, cargo-linux-64, cargo-mac-32, cargo-mac-64, cargo-win-32, cargo-win-64