* First shot at updating changelog for 16.0

**what is the change?:**
Added an 'unreleased' section to the changelog with info from facebook#10294

**why make this change?:**
To get things set for the 16.0 release.

**test plan:**
Visual inspection

**issue:**
facebook#8854

* Fix typos and formatting errors in changelog

* Add requestAnimationFrame and remove "New helpful warnings"

**what is the change?:**
In response to helpful code review -
- Add mention of dependency on `requestAnimationFrame` and need to
  polyfill that as well as `Map` and `Set`
- Remove "New helpful warnings" section; it was incomplete, and there
  are so many new and updated warnings that it might not be reasonable
  to cover them in the changelog.

**why make this change?:**
Accuracy

**test plan:**
Visual inspection

**issue:**
issue facebook#8854

* Improve wording

* Improve wording and fix missing links

* Add backticks to file names & code; wording tweak

* Break "Major Changes" into "New Feature" and "Breaking Changes"

* Add server side render changes to 16.0 changelog

* Change gist link from mine to gaearons

* Add note about returning fragments

* fix misc nits

* Misc. formatting/wording fixes to changelog

**what is the change?:**
Thanks to the kind code review comments of @gaearon and @nhunzaker we
have
- removed the non-deterministic bold styling on some bullet points
- improved wording of the bullet points for portals, attribute whitelist
  changes, and server rendering changes
- Add note about error boundaries including a breaking change to error
  handling behavior.
- punctuation and capitalization fixes

**why make this change?:**
Clarity and correctness

**test plan:**
Visual inspection

**issue:**
facebook#8854

* fix broken link

…de on correct document (facebook#10723)

*  Add a changelog for elements having the same key

* Reword

* Update DOM warning wording and link

* Consistently use "Invalid" for known misspellings

Did find and replace in TextMate.

```
find: (?:( \*)( ))?Copyright (?:\(c\) )?(\d{4})\b.+Facebook[\s\S]+(?:this source tree|the same directory)\.$
replace: $1$2Copyright (c) $3-present, Facebook, Inc.\n$1\n$1$2This source code is licensed under the MIT license found in the\n$1$2LICENSE file in the root directory of this source tree.
```

Only remaining references:

```
docs/_posts/2014-10-28-react-v0.12.md
51:You can read the full text of the [LICENSE](https://github.com/facebook/react/blob/master/LICENSE) and [`PATENTS`](https://github.com/facebook/react/blob/master/PATENTS) files on GitHub.

docs/_posts/2015-04-17-react-native-v0.4.md
20:* **Patent Grant**: Many of you had concerns and questions around the PATENTS file. We pushed [a new version of the grant](https://code.facebook.com/posts/1639473982937255/updating-our-open-source-patent-grant/).

src/__mocks__/vendor/third_party/WebComponents.js
8: * subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
```

* Add ReactTestRenderer documentations

* Add TestRenderer documentations

* TestRenderer is not experiment

* Add a link for jsdom

* Use ES Modules syntax

* Twaek

* Add a Link component

* Use Jest assertions

* Move a documentation for createNodeMock to Idea section

* Renamed

* Tweak

* Rename

* More explicit

* Add a usecase for createNodeMock

* Doc updates for React 16 + blog post

* Add link to Sophie's post

* Fix React links on the website

* Fix code editor

* Fix code editor, attempt 2

* Doc change for prevContext removal in CDU

Ref: facebook#8631

* Minor rewording

* Update Portals Documentation

Correct some grammar to be more explicit and clear. Update example CodePen to better match code found in documentation. Update code example styles to match other code examples (ie. 'State and Lifecycle', 'Handling Events').

* Clean up comment to be accurate to example

There was a small comment overlooked when reviewing the documentation. This fixes it to be accurate to the example as well as grammatically correct.

* Update portals.md

* More fixes

The proposal for property initializers is called [Public Class Fields](https://tc39.github.io/proposal-class-public-fields/) now (part of the combined [Class Fields](https://github.com/tc39/proposal-class-fields) proposal).