Update release notes

Author: marshallpierce

RELEASE-NOTES.md

@@ -1,5 +1,8 @@
 # 0.20.0
 
+## 0.20.0-alpha.1
+
+### Breaking changes
 - Extended the `Config` concept into the `Engine` abstraction, allowing the user to pick different encoding / decoding implementations.
   - What was formerly the only algorithm is now the `FastPortable` engine, so named because it's portable (works on any CPU) and relatively fast.
   - This opens the door to a portable constant-time implementation ([#153](https://github.com/marshallpierce/rust-base64/pull/153), presumably `ConstantTimePortable`?) for security-sensitive applications that need side-channel resistance, and CPU-specific SIMD implementations for  more speed.