Update package docs (#38)

* Update README.md

* Add exec guidance

* Add io guidance

* Add tool-cache guidance

* Readability

* Readability

* Readability

* Nit

* Nit

* Nit

packages/core/README.md

@@ -4,4 +4,83 @@
 
 ## Usage
 
-See [src/core.ts](src/core.ts).
+#### Inputs/Outputs
+
+You can use this library to get inputs or set outputs:
+
+```
+const core = require('@actions/core');
+
+const myInput = core.getInput('inputName', { required: true });
+
+// Do stuff
+
+core.setOutput('outputKey', 'outputVal');
+```
+
+#### Exporting variables/secrets
+
+You can also export variables and secrets for future steps. Variables get set in the environment automatically, while secrets must be scoped into the environment from a workflow using `{{ secret.FOO }}`. Secrets will also be masked from the logs:
+
+```
+const core = require('@actions/core');
+
+// Do stuff
+
+core.exportVariable('envVar', 'Val');
+core.exportSecret('secretVar', variableWithSecretValue);
+```
+
+#### PATH Manipulation
+
+You can explicitly add items to the path for all remaining steps in a workflow:
+
+```
+const core = require('@actions/core');
+
+core.addPath('pathToTool');
+```
+
+#### Exit codes
+
+You should use this library to set the exit code for your action:
+
+```
+const core = require('@actions/core');
+
+try {
+  if (work to do) {
+    // Do work
+  } else {
+    // Set neutral indicates that the action terminated but did not fail (aka there was no work to be done)
+    core.setNeutral();
+  }
+}
+catch (err) {
+  // setFailed logs the message and sets a failing exit code
+  core.setFailed(`Action failed with error ${err}`);
+}
+
+```
+
+#### Logging
+
+Finally, this library provides some utilities for logging:
+
+```
+const core = require('@actions/core');
+
+const myInput = core.getInput('input');
+try {
+  core.debug('Inside try block');
+  
+  if (!myInput) {
+    core.warning('myInput wasnt set');
+  }
+  
+  // Do stuff
+}
+catch (err) {
+  core.error('Error ${err}, action may still succeed though');
+}
+```