Add a CHANGELOG.md.

Author: dmitrizagidulin

CHANGELOG.md

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+## v2.0
+
+### Changed
+- (BREAKING) Moved package.json `main:` to `src/` instead of `lib/` (no longer
+  transpiling to ES5).
+- Remove dependency on `json-document`.
+- Replaced `@trust/webcrypto` with `isomorphic-webcrypto`.
+- Updated various dependencies to latest.
+- Switched to a maintained `text-encoder` lib (`@sinonjs/text-encoder`).
+
+## v1.8 - 2018-08-24
+
+- Forked into the Solid github org.