Implement 0 as no timeout for TaskRun

If the timeout is set to 0, there is no timeout for the TaskRun.
If the timeout is empty, the default timeout 60 mins will be applied.

docs/taskruns.md

@@ -48,8 +48,10 @@ following fields:
   - [`inputs`] - Specifies [input parameters](#input-parameters) and
     [input resources](#providing-resources)
   - [`outputs`] - Specifies [output resources](#providing-resources)
-  - `timeout` - Specifies timeout after which the `TaskRun` will fail. Defaults
-    to 60 minutes.
+  - `timeout` - Specifies timeout after which the `TaskRun` will fail. If the value of
+    `timeout` is empty, the default timeout will be applied. If the value is set to 0,
+    there is no timeout. You can also follow the instruction [here](#Configuring-default-timeout)
+    to configure the default timeout.
   - [`nodeSelector`] - a selector which must be true for the pod to fit on a
     node. The selector which must match a node's labels for the pod to be
     scheduled on that node. More info:
@@ -145,6 +147,13 @@ spec:
               value: https://github.com/pivotal-nader-ziada/gohelloworld
 ```
 
+### Configuring Default Timeout
+
+You can configure the default timeout by changing the value of `default-timeout-minutes`
+in [`config/config-defaults.yaml`](./../config/config-defaults.yaml). The default timeout
+is 60 minutes, if `default-timeout-minutes` is not available. There is no timeout by
+default, if `default-timeout-minutes` is set to 0.
+
 ### Service Account
 
 Specifies the `name` of a `ServiceAccount` resource object. Use the