Add notes on pre/post hook behaviour

Author: j4mie

README.md

@@ -113,7 +113,7 @@ JOBS = {
 ```
 
 #### Pre & Post Task Hooks
-You can also run pre task or post task hooks, which happen in the normal processing of your `Job` instances and are executed in the worker process.
+You can also run pre task or post task hooks, which happen in the normal processing of your `Job` instances and are executed inside the worker process.
 
 Both pre and post task hooks receive your `Job` instance as their only argument. Here's an example:
 
@@ -122,7 +122,7 @@ def my_pre_task_hook(job):
     ...  # configure something before running your task
 ```
 
-To ensure these hooks gets run, simply add a `pre_task_hook` or `post_task_hook` key (or both, if needed) to your job config like so:
+To ensure these hooks are run, simply add a `pre_task_hook` or `post_task_hook` key (or both, if needed) to your job config like so:
 
 ```python
 JOBS = {
@@ -134,6 +134,12 @@ JOBS = {
 }
 ```
 
+Notes:
+
+* If the `pre_task_hook` fails (raises an exception), the task function is not run, and django-db-queue behaves as if the task function itself had failed: the failure hook is called, and the job is goes into the `FAILED` state.
+* The `post_task_hook` is always run, even if the job fails. In this case, it runs after the `failure_hook`.
+* If the `post_task_hook` raises an exception, this is logged but the the job is **not marked as failed** and the failure hook does not run. This is because the `post_task_hook` might need to perform cleanup that always happens after the task, no matter whether it succeeds or fails.
+
 
 ### Start the worker