Update package docs (#38)

* Update README.md

* Add exec guidance

* Add io guidance

* Add tool-cache guidance

* Readability

* Readability

* Readability

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README.md

@@ -1,7 +1,60 @@
 # `@actions/exec`
 
-> Functions necessary for running tools on the command line
-
 ## Usage
 
-See [src/exec.ts](src/exec.ts).
+#### Basic
+
+You can use this package to execute your tools on the command line in a cross platform way:
+
+```
+const exec = require('@actions/exec');
+
+await exec.exec('node index.js');
+```
+
+#### Args
+
+You can also pass in arg arrays:
+
+```
+const exec = require('@actions/exec');
+
+await exec.exec('node', ['index.js', 'foo=bar']);
+```
+
+#### Output/options
+
+Capture output or specify [other options](https://github.com/actions/toolkit/blob/d9347d4ab99fd507c0b9104b2cf79fb44fcc827d/packages/exec/src/interfaces.ts#L5):
+
+```
+const exec = require('@actions/exec');
+
+const myOutput = '';
+const myError = '';
+
+const options = {};
+options.listeners = {
+  stdout: (data: Buffer) => {
+    myOutput += data.toString();
+  },
+  stderr: (data: Buffer) => {
+    myError += data.toString();
+  }
+};
+options.cwd = './lib';
+
+await exec.exec('node', ['index.js', 'foo=bar'], options);
+```
+
+#### Exec tools not in the PATH
+
+You can use it in conjunction with the `which` function from `@actions/io` to execute tools that are not in the PATH:
+
+```
+const exec = require('@actions/exec');
+const io = require('@actions/io');
+
+const pythonPath: string = await io.which('python', true)
+
+await exec.exec(`"${pythonPath}"`, ['main.py']);
+```