Adding --check to run a serials network test against GitHub or GHES. (#900)

* add --check.

docs/checks/nodejs.md

@@ -0,0 +1,30 @@
+# Node.js Connection Check
+
+## What is this check for?
+
+Make sure the built-in node.js has access to GitHub.com or GitHub Enterprise Server.
+
+The runner carries it's own copy of node.js executable under `<runner_root>/externals/node12/`.
+
+All javascript base Actions will get executed by the built-in `node` at `<runner_root>/externals/node12/`.
+
+> Not the `node` from `$PATH`
+
+## What is checked?
+
+- Make HTTPS GET to https://api.github.com or https://myGHES.com/api/v3 using node.js, make sure it gets 200 response code.
+
+## How to fix the issue?
+
+### 1. Check the common network issue
+  
+  > Please check the [network doc](./network.md)
+
+### 2. SSL certificate related issue
+
+  If you are seeing `Https request failed due to SSL cert issue` in the log, it means the `node.js` can't connect to the GitHub server due to SSL handshake failure.
+  > Please check the [SSL cert doc](./sslcert.md)
+  
+## Still not working?
+
+Contact GitHub customer service or log an issue at https://github.com/actions/runner if you think it's a runner issue.