Update ERC20 Guide link (was 404)

[wanderingstan]

Link for "ERC20 Guide" on Token Readme is giving 404.

https://github.com/OpenZeppelin/openzeppelin-solidity/blob/master/contracts/tokens.md#constructing-a-nice-erc20-token

Looks like tokens.md has been moved to the ./docs dir. The # link matched the section title so I assume this is where it was meant to go.

EDIT: No issue found for this. #1790 is also about broken links, but not this particular one.

[frangio]

Hm, unfortunately the problem here is that the link is meant to work in the published site, where it does currently work, and not in the GitHub preview.

Merging this PR would break the published site, so I'll close it. But I understand this is not ideal. I'm thinking perhaps the documentation generator that parses these files should adjust the links so they work in both places.

[wanderingstan]

Oh, I didn't even realize there was a "real" published site and that you're treating the repo as merely a preview.

Perhaps a stop-gap solution would be to put a standard note at the top of the all repo readme's saying something like "Please view our full documentation at https://docs.openzeppelin.org/"? This line could then be easily stripped by the doc generator.