Added changelog. (#1555)

* Added changelog.

* Added PR template note.

* Update .github/PULL_REQUEST_TEMPLATE.md

Co-Authored-By: nventuro <nicolas.venturo@gmail.com>

* Update PULL_REQUEST_TEMPLATE.md

* Update CHANGELOG.md

Co-Authored-By: nventuro <nicolas.venturo@gmail.com>

* Update CHANGELOG.md

Co-Authored-By: nventuro <nicolas.venturo@gmail.com>

* Apply suggestions from code review

Co-Authored-By: nventuro <nicolas.venturo@gmail.com>

* Added more headers and sub-headers.

* Added deprecations category.

* Added PR links.

* Added PausableCrowdsale.

* Addressed review comments.

* Update CHANGELOG.md

Co-Authored-By: nventuro <nicolas.venturo@gmail.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -15,6 +15,7 @@ Fixes #
   - reviewed the OpenZeppelin Contributor Guidelines
     (https://github.com/OpenZeppelin/openzeppelin-solidity/blob/master/CONTRIBUTING.md),
   - added tests where applicable to test new functionality,
-  - made sure that your contracts are well-documented, and
-  - run the JS/Solidity linters and fixed any issues (`npm run lint:fix`).
+  - made sure that your contracts are well-documented,
+  - run the JS/Solidity linters and fixed any issues (`npm run lint:fix`), and
+  - updated the changelog, if applicable.
 -->

CHANGELOG.md

+# Changelog
+
+## 2.1.0 (unreleased)
+
+### New features:
+ * `WhitelistCrowdsale`: a crowdsale where only whitelisted accounts (`WhitelistedRole`) can purchase tokens. Adding or removing accounts from the whitelist is done by whitelisters (`WhitelisterRole`). Similar to the pre-2.0 `WhitelistedCrowdsale`. ([#1525](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1525))
+ * `RefundablePostDeliveryCrowdsale`: replacement for `RefundableCrowdsale` (deprecated, see below) where tokens are only granted once the crowdsale ends (if it meets its goal). ([#1543](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1543))
+ * `PausableCrowdsale`: allows for pausers (`PauserRole`) to pause token purchases. Other crowdsale operations (e.g. withdrawals and refunds, if applicable) are not affected. ([#832](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/832))
+ * `ERC20`: `transferFrom` and `_burnFrom ` now emit `Approval` events, to represent the token's state comprehensively through events. ([#1524](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1524))
+ * `ERC721`: added `_burn(uint256 tokenId)`, replacing the similar deprecated function (see below). ([#1550](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1550))
+ * `ERC721`: added `_tokensOfOwner(address owner)`, allowing to internally retrieve the array of an account's owned tokens. ([#1522](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1522))
+
+### Improvements:
+ * The compiler version required by `Array` was behind the rest of the libray so it was updated to `v0.4.24`. ([#1553](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1553))
+ * Now conforming to a 4-space indentation code style. ([1508](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1508))
+ * `ERC20`: more gas efficient due to removed redundant `require`s. ([#1409](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1409))
+ * `ERC721`: fixed a bug that prevented internal data structures from being properly cleaned, missing potential gas refunds. ([#1539](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1539) and [#1549](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1549))
+ * `ERC721`: general gas savings on `transferFrom`, `_mint` and `_burn`, due to redudant `require`s and `SSTORE`s. ([#1549](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1549))
+
+### Bugfixes:
+
+### Breaking changes:
+
+### Deprecations:
+ * `ERC721._burn(address owner, uint256 tokenId)`: due to the `owner` parameter being unnecessary. ([#1550](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1550))
+ * `RefundableCrowdsale`: due to trading abuse potential on crowdsales that miss their goal. ([#1543](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1543))