Add Changelog and Docs for --parallel

Author: andreynering

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## Unreleased
+
+- Add `--parallel` flag (alias `-p`) to run tasks given by the command line in
+  parallel
+  ([#266](https://github.com/go-task/task/pull/266)).
+
 ## v2.7.1 - 2019-11-10
 
 - Fix error being raised when `exit 0` was called

docs/usage.md

@@ -193,6 +193,9 @@ tasks:
 If there is more than one dependency, they always run in parallel for better
 performance.
 
+> You can also make the tasks given by the command line run in parallel by
+> using the `--parallel` flag (alias `-p`). Example: `task --parallel js css`.
+
 If you want to pass information to dependencies, you can do that the same
 manner as you would to [call another task](#calling-another-task):