Fix Celery URL (donnemartin#486)

Author: sharpcodex

README.md

@@ -1350,7 +1350,7 @@ The user is not blocked and the job is processed in the background.  During this
 
 Tasks queues receive tasks and their related data, runs them, then delivers their results.  They can support scheduling and can be used to run computationally-intensive jobs in the background.
 
-**Celery** has support for scheduling and primarily has python support.
+**[Celery](https://docs.celeryproject.org/en/stable/)** has support for scheduling and primarily has python support.
 
 ### Back pressure