docs: add changelog

ChangeLog

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+Version 2.0.7
+* stable release, optional update
+* Bitcoin Cash cashaddr support
+* Zcash Overwinter hardfork support
+* NEM support
+* Lisk support
+* Show warning on home screen if PIN is not set
+* Support for new coins:
+  - Decred, Bitcoin Private, Fujicoin, Groestlcoin, Vertcoin, Viacoin, Zcoin
+* Support for new Ethereum networks:
+  - EOS Classic, Ethereum Social, Ellaism, Callisto, EtherGem, Wanchain
+* Support for 500+ new Ethereum tokens
+
+Version 2.0.6
+* stable release, optional update
+* fix layout for Ethereum transactions
+* fix public key generation for SSH and GPG
+* add special characters to passphrase keyboard
+
+Version 2.0.5
+* stable release, required update
+* first public release