changelog for 0.20.4: add more PR changes

Many more significant changes have been made since version 0.20.3 was
released. This commit adds more of those changes to the changelog for
version 0.20.4, and removes the `-rc1` release candidate designation.

Signed-off-by: Geoffrey M. Oxberry <[email protected]>

CHANGELOG.md

@@ -6,9 +6,26 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
-## [0.20.4-rc1]
+## [0.20.4]
+### Added
+- `lading.running` gauge metric that submits a value of 1 as long as the main
+  `select!` of lading is executing.
+- `target.running` gauge metric that submits a value of 1 as long as the target
+  is running.
+
 ### Changed
 - Dogstatsd payload structs now have public fields.
+- Capture file descriptor is flushed as soon as current lines are written.
+- Renamed `Shutdown` data type to `Phase` so that this data type can be used as
+  a signal to control `lading` behavior over distinct stages of load generation
+  (e.g., warmup, shutdown).
+- Prometheus telemetry and Go expvar target metrics are no longer emitted during
+  `lading`'s warmup ("experimental") phase.
+
+### Fixed
+- Refactors the main `select!` in `lading/src/bin/lading.rs` to loop over the
+  select to address a potential early-termination bug when the set of generators
+  are empty.
 
 ## [0.20.3]
 ### Added