Release tracking `0.2.0`

[ValuedMammal]

Tracking work that is planned for a 0.2.0 release of bdk_tx. The list includes potentially API breaking features. Other non-breaking features, documentation, and example code changes can be incorporated as they become ready.

• Is there a way to specify the absolute fee of a transaction? #26 aka FeePolicy
• feat: enable random anti-fee sniping #5 pending ongoing discussion
• Make ChangePolicyType more flexible #29
• Add CHANGELOG.md Release 0.2.0 #35