Clarify README wrt ephemeral port in embedded mode (#1524)

If we don't set port=0, the default port 8899 is used. So I propose to clarify the documentation.

Author: sbidoul

README.md

@@ -1725,7 +1725,7 @@ In embedded mode, you can access this port.  Example:
 import proxy
 
 if __name__ == '__main__':
-  with proxy.Proxy() as p:
+  with proxy.Proxy(port=0) as p:
     print(p.flags.port)
     proxy.sleep_loop()
 ```

docs/changelog-fragments.d/1524.doc.md

@@ -0,0 +1 @@
+Clarify documentation for using ephemeral ports in embedded mode.