feat: add ability to specify which vars are required (#1204)

docs/docs/usage.md

@@ -876,6 +876,48 @@ tasks:
       - sleep 5 # long operation like installing packages
 ```
 
+### Ensuring required variables are set
+
+If you want to check that certain variables are set before running a task then
+you can use `requires`. This is useful when might not be clear to users which
+variables are needed, or if you want clear message about what is required. Also
+some tasks could have dangerous side effects if run with un-set variables.
+
+Using `requires` you specify an array of strings in the `vars` sub-section 
+under `requires`, these strings are variable names which are checked prior to 
+running the task. If any variables are un-set the the task will error and not 
+run. 
+
+Environmental variables are also checked.
+
+Syntax:
+
+```yaml
+requires: 
+  vars: [] # Array of strings
+```
+
+:::note
+
+Variables set to empty zero length strings, will pass the `requires` check.
+
+:::
+
+Example of using `requires`:
+
+```yaml
+version: '3'
+
+tasks:
+  docker-build:
+    cmds:
+      - 'docker build . -t {{.IMAGE_NAME}}:{{.IMAGE_TAG}}'
+
+    # Make sure these variables are set before running
+    requires: 
+      vars: [IMAGE_NAME, IMAGE_TAG]
+```
+
 ## Variables
 
 When doing interpolation of variables, Task will look for the below. They are