dev: update README

The information here had become out of date.

Release note: None

pkg/cmd/dev/README.md

@@ -1,26 +1,12 @@
-WARNING: `dev` isn't feature-complete, and the migration to Bazel at Cockroach
-is still in-progress. Proceed at your own risk :)
+WARNING: The migration to Bazel at Cockroach is still in-progress.
+Proceed at your own risk :)
 
-Dev is a general-purpose dev tool for folks working on `cockroach`.
+`dev` is a general-purpose tool for folks working on `cockroach`. Unlike
+builds and tests performed w/ `make`, everything `dev` does is driven by
+Bazel as a build system.
 
-    $ dev -h
-      <...>
-      Usage:
-        dev [command]
+You can use the script `./dev` at top-level instead of building `dev`
+ahead of time. This script just builds `dev` and immediately runs it
+with the supplied arguments.
 
-      Available Commands:
-        bench       Run the specified benchmarks
-        build       Build the specified binaries
-        generate    Generate the specified files
-        lint        Run the specified linters
-        test        Run the specified tests
-
-      Flags:
-        -h, --help      help for dev
-        -v, --version   version for dev
-
-      Use "dev [command] --help" for more information about a command.
-
-To update the testdata files:
-
-    $ go test -run TestDatadriven -rewrite [-record] [-from-checkout=<path-to-crdb-checkout>]
+Run `./dev help` to see what `dev` can do!