Add some documentation on TaskRun usage with parameters.

docs/task-parameters.md

@@ -0,0 +1,49 @@
+## Task Parameters
+
+Tasks can declare input parameters that must be supplied to the task during a TaskRun.
+Some example use-cases of this include:
+
+* A task that needs to know what compilation flags to use when building an application.
+* A task that needs to know what to name a built artifact.
+* A task that supports several different strategies, and leaves the choice up to the other.
+
+### Usage
+
+The following example shows how Tasks can be parameterized, and these parameters can be passed to the `Task` from a `TaskRun`.
+
+Input parameters in the form of `${inputs.foo}` are replaced inside of the buildSpec.
+
+The following `Task` declares an input parameter called 'flags', and uses it in the `buildSpec.steps.args` list.
+
+```yaml
+apiVersion: pipeline.knative.dev/v1alpha1
+kind: Task
+metadata:
+  name: task-with-parameters
+spec:
+    inputs:
+        params:
+        - name: flags
+          value: string
+    buildSpec:
+        steps:
+        - name: build
+          image: my-builder
+          args: ['build', '--flags=${inputs.flags}']
+```
+
+The following `TaskRun` supplies a value for `flags`:
+
+```yaml
+apiVersion: pipeline.knative.dev/v1alpha1
+kind: TaskRun
+metadata:
+  name: run-with-parameters
+spec:
+    taskRef:
+        name: task-with-parameters
+    inputs:
+        params:
+        - name: 'flags'
+          value: 'foo=bar,baz=bat'
+```