Update README for the release of OpenZeppelin Contracts Ethereum Package v3.0

Change guides from being Contracts to all of docs

Minor wording changes
Update links
Update samples

(cherry picked from commit fd981ad3)

* Making IERC721Receiver an interface

* Update IERC721Receiver.sol

Co-authored-by: Nicolás Venturo <nicolas.venturo@gmail.com>
(cherry picked from commit 78dc3773)

* remove ethpm.json

* update releasing document

* fix release documentation to be more realistic

* Add baseURI to ERC721MinterPauser constructor

* Add tokenURI to mint

* Remove tokenId and tokenURI from ERC721 deploy ready

* Rename ERC721MinterPauser to ERC721MinterAutoID, remove Pausable mechanisms

* Restore pausing to ERC721

* Rename deploy ready to presets

* Rename ERC721 preset

* Improve Hooks docs

* Improve Utils docs

* Apply suggestions from code review

Co-Authored-By: Francisco Giordano <frangio.1@gmail.com>

* Add enumerable code samples

* Remove import statement from example

Co-authored-by: Francisco Giordano <frangio.1@gmail.com>

Bumps [acorn](https://github.com/acornjs/acorn) from 6.4.0 to 6.4.1. **This update includes security fixes.**
- [Release notes](https://github.com/acornjs/acorn/releases)
- [Commits](https://github.com/acornjs/acorn/compare/6.4.0...6.4.1)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

Bumps [solhint](https://github.com/protofire/solhint) from 3.0.0-rc.7 to 3.0.0-rc.8.
- [Release notes](https://github.com/protofire/solhint/releases)
- [Changelog](https://github.com/protofire/solhint/blob/master/CHANGELOG.md)
- [Commits](https://github.com/protofire/solhint/compare/v3.0.0-rc.7...v3.0.0-rc.8)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

Bumps [@openzeppelin/test-environment](https://github.com/OpenZeppelin/openzeppelin-test-environment) from 0.1.3 to 0.1.4.
- [Release notes](https://github.com/OpenZeppelin/openzeppelin-test-environment/releases)
- [Changelog](https://github.com/OpenZeppelin/openzeppelin-test-environment/blob/master/CHANGELOG.md)
- [Commits](https://github.com/OpenZeppelin/openzeppelin-test-environment/compare/v0.1.3...v0.1.4)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

Bumps [@openzeppelin/cli](https://github.com/OpenZeppelin/openzeppelin-sdk) from 2.8.1 to 2.8.2.
- [Release notes](https://github.com/OpenZeppelin/openzeppelin-sdk/releases)
- [Commits](https://github.com/OpenZeppelin/openzeppelin-sdk/compare/v2.8.1...v2.8.2)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

* Remove isConstructor requirement from _setupRole

* Remove isConstructor requirement from _setupDecimals

* Update contracts/access/AccessControl.sol

Co-Authored-By: Francisco Giordano <frangio.1@gmail.com>

Co-authored-by: Francisco Giordano <frangio.1@gmail.com>

* Fix deprecation warnings

* Workaround linter bug

* rename Strings.fromUint256 to Strings.toString

* add changelog entry

* fix conflict with js toString method

* Improve ERC20/721 Pausable docs

* Add ERC20Pausable mint and burn tests

* Add ERC721Pausable mint and burn tests

* Add _beforeTransfer hook in ERC777 to mint and burn

* provide 'automatic' token URI by appending the token ID to the base URI, if a base is set but no token-specific URI is available

* make the three cases more explicit, avoid else after return

* adjust comments to reflect reality