vdk-plugins: add connection hook activity diagram (#1786)

I was playing around (learning how to write PlantUML activity diagrams)
and wrote one about how PEP 249 operations and conneciton hooks work.

In my opinion, a picture is worth a thousand words and the current
documentation for connection hooks is a bit too text-heavy without
showing enough.

I think this would improve understandability of how connection hooks
work

---------

Signed-off-by: Antoni Ivanov <aivanov@vmware.com>

Author: antoniivanov

projects/vdk-plugins/README.md

@@ -125,6 +125,8 @@ Sequence of evaluation:
 
 Check out the [connection hook spec](../vdk-core/src/vdk/api/plugin/connection_hook_spec.py) documentation for more details.
 
+[![connection-hooks-activity-diagram](https://user-images.githubusercontent.com/2536458/228570184-4fba653c-dd6a-4a6d-80b3-bee83beb85e6.svg)](https://www.plantuml.com/plantuml/uml/ZPDFQnin4CNl-XG3kTY7-51wgGqb4BiKcaBwf-cXBKQMnbwrMib8sgI1_V0PQOMrcrs7-EBTVK-_DoEDhdpWBZIrPe8VWx86lbVAWrJyu7WDlh8FzCO3tt6FSBkvXJTltu4zekFHxU51XGhkrf_WsXg38Y4-MllFBnZJU40ZsKtwMx8M3WxHG7lYVCPGMGdThoN3JZT8-WGlwaJ9ICRQ7nvTorBv360f7FA084whLgmbJ1krduuVYJTMBevSOofgMUH5PipcP5pdrXDdu-b5Ar_rOwBm5KFZJEyhsDrVUbhbCli5DivRjpeVdlHnTexev0dyvZ-AXlZVljo0i7NDZNmUafOki3DIGjcWEwwLv07BmQQrMXsg48zaANVRqjpsFjkt9tka4MUDmhhNSsIqZpcb6Ug27r2-4fSxAxJnBcPmsI6rXnawPzqSGeK6Pe_ev-Iy_7NXKFwvV4_FUPlE1piKzXxTC7Z8Y3dPXdASbKweSwBs23DZep9ab4hYF715lbJwQiBfWpr6c95gpmfo69RKwJbpw1iTOjdSFAwOvZlKu9AsxRJUx7t082gGX8aB3AB4CyEtZqvhQFf-c_wdcbAUV-DQdxq6oO0oPVPlmRMjQnKWE6uyxsvYgUY52nzNZSF6j46MjhxSvwbkHNICi5cDMsmR9z3JaqOIvPXUXko5wgTJYcCoAGt85HhPrFe9)
+
 ## Public interfaces
 
 Any backwards compatibility guarantees apply only to public interfaces.