docs(NODE-4923): make note of drawbacks of the legacy driver (#13)

Author: baileympearson

readme.md

@@ -18,6 +18,13 @@ If callback usage is needed for any new APIs, we would suggest using `.then`/`.c
 
 The main driver package `mongodb` will be dropping optional callback support in the next major version (v5) in favor of `async`/`await` syntax.
 
+### Drawbacks
+
+- The legacy driver wraps the native driver, which may lead to a slight performance penalty.
+- As discussed in the versioning section, it is recommended that `mongodb-legacy` replace the direct dependency on `mongodb`.  This allows for
+the legacy driver to automatically pull in features and fixes from the native driver.  However, this also removes control
+of which version of mongodb is installed.  If users wish to control the version of mongodb directly, the lockfile will need edited manually.
+
 ### Example usage of equivalent callback and promise usage
 
 ```ts