docs(core): update core package readme

Add description of getBooleanInput function.

packages/core/README.md

@@ -16,11 +16,11 @@ import * as core from '@actions/core';
 
 #### Inputs/Outputs
 
-Action inputs can be read with `getInput`.  Outputs can be set with `setOutput` which makes them available to be mapped into inputs of other actions to ensure they are decoupled.
+Action inputs can be read with `getInput` while the return type is `string`, and the inputs of the boolean type in the [YAML specification](https://yaml.org/type/bool.html) can be read with `getBooleanInput` while the return type is `boolean`. Outputs can be set with `setOutput` which makes them available to be mapped into inputs of other actions to ensure they are decoupled.
 
 ```js
-const myInput = core.getInput('inputName', { required: true });
-
+const myInput = core.getInput('inputName', { required: true })
+const myBooleanInput = core.getBooleanInput('booleanInputName');
 core.setOutput('outputKey', 'outputVal');
 ```