Prepare v0.20.0 changelog

Signed-off-by: Thane Thomson <[email protected]>

.changelog/v0.20.0/summary.md

@@ -0,0 +1,18 @@
+This release's number is bumped up to v0.20.0 due to one breaking change in the
+`tendermint-p2p` crate's error naming conventions to make them more idiomatic
+(see [#898](https://github.com/informalsystems/tendermint-rs/pull/898)).
+
+Also, since nobody was really making use of the Light Node, we decided to remove
+its crate from the repo for now. If anyone needs it back, please contact us and
+we'll restore it (although, we are considering migrating any and all binaries to
+their own repositories in future to separate framework-level concerns from
+operational ones).
+
+The `tendermint-p2p` crate is still undergoing a significant amount of work
+(thanks to @xla and @melekes!). A lot's been done and we're in the process of
+finalizing this new architecture, which will form the basis for future work
+towards building a Tendermint full node/validator in Rust. More on this in
+future releases.
+
+Other than that, this release mainly contains various small bug fixes,
+improvements and dependency updates.