Adding concurrency control in our updateMovieHandler and documenting it.

README.md

@@ -44,6 +44,14 @@ There are two comon approaches to doing this :
 
 From an HTTP semantics point of view, using headers to convey the API version is the 'purer' approach. But from a user-experience point of view, using a URL prefix is arguably better. It makes it possible for developers to see which version of the API is being used at a glance, and it also means that the API can still be explored using a regular web browser (which is harder if custom headers are required).
 
+## Concurrent request to test data race
+
+In our 'updateMovieHandler' we prevent data race. To test if everything is going as we want, we can try multiple request at the same time with this command :
+
+```bash
+xargs -I % -P8 curl -X PATCH -d '{"runtime": "97 mins"}' "localhost:4000/v1/movies/4" < <(printf '%s\n' {1..8})
+```
+
 ## SQL Migrations
 
 The first thing we need to do is generate a pair of _migration files_ using the **migrate create** command :