doc(netxlite): revamp the documentation (#523)

Part of https://github.com/ooni/probe-cli/pull/506. In parallel with
tutorials, we also need to make sure we have good documentation.

internal/README.md

@@ -1,3 +1,19 @@
 # Directory github.com/ooni/probe-cli/internal
 
 This directory contains private Go packages.
+
+As a reminder, you can always check the Go documentation of
+a package by using
+
+```bash
+go doc -all ./internal/$package
+```
+
+where `$package` is the name of the package.
+
+Some notable packages:
+
+- [netxlite](netxlite) is the underlying networking library;
+
+- [tutorial](tutorial) contains tutorials on writing new experiments,
+using measurements libraries, and networking code.