Add hability silent all tasks

By add `silent: true` at the root of the Taskfile.

docs/usage.md

@@ -674,7 +674,7 @@ With silent mode on, the below will be print instead:
 Print something
 ```
 
-There's three ways to enable silent mode:
+There are four ways to enable silent mode:
 
 * At command level:
 
@@ -700,6 +700,19 @@ tasks:
     silent: true
 ```
 
+* Globally at Taskfile level:
+
+```yaml
+version: '2'
+
+silent: true
+
+tasks:
+  echo:
+    cmds:
+      - echo "Print something"
+```
+
 * Or globally with `--silent` or `-s` flag
 
 If you want to suppress STDOUT instead, just redirect a command to `/dev/null`: