Add link to pgx presentation to README.md

Author: jackc

README.md

@@ -86,6 +86,10 @@ It is also possible to use the `database/sql` interface and convert a connection
 
 See CONTRIBUTING.md for setup instructions.
 
+## Architecture
+
+See the presentation at Golang Estonia, [PGX Top to Bottom](https://www.youtube.com/watch?v=sXMSWhcHCf8) for a description of pgx architecture.
+
 ## Supported Go and PostgreSQL Versions
 
 pgx supports the same versions of Go and PostgreSQL that are supported by their respective teams. For [Go](https://golang.org/doc/devel/release.html#policy) that is the two most recent major releases and for [PostgreSQL](https://www.postgresql.org/support/versioning/) the major releases in the last 5 years. This means pgx supports Go 1.20 and higher and PostgreSQL 11 and higher. pgx also is tested against the latest version of [CockroachDB](https://www.cockroachlabs.com/product/).