doc(tutorial): explain the big picture (#582)

Each tutorial provides you a piece of knowledge. Add some text in
the main readme file explaining the big picture to the reader.

Closes https://github.com/ooni/probe/issues/1819

internal/tutorial/README.md

@@ -8,11 +8,28 @@ real OONI code, it should always be up to date.
 
 ## Index
 
-- [Rewriting the torsf experiment](experiment/torsf/)
+- [Rewriting the torsf experiment](experiment/torsf/): this tutorial
+explains to you how to write a simple experiment. After reading it, you
+will understand the interfaces between an experiment and the OONI
+core. What this tutorial does not teach you, though, is how
+to tell the OONI core about this experiment. To see how to do that,
+you should check how we do that in [internal/engine/allexperiments.go](
+../engine/allexperiments.go).
 
-- [Using the measurex package to write network experiments](measurex)
+- [Using the measurex package to write network experiments](measurex): this
+tutorial explains to you how to use the `measurex` library to write networking
+code that generates measurements using the OONI data format. You will learn
+how to perform DNS, TCP, TLS, QUIC, HTTP, HTTPS, and HTTP3 measurements.
 
-- [Low-level networking using netxlite](netxlite)
+- [Low-level networking using netxlite](netxlite): this tutorial introduces
+you to the `netxlite` networking library. This is the underlying library
+used by `measurex` as well as by many other libraries inside OONI. You need
+to know about this library to contribute to `measurex` as well as to other
+parts of the OONI core that perform network operations.
+
+Therefore, after reading these tutorials, you should have a better
+understanding of how an experiment interacts with the OONI core, as
+well as of which libraries you can use to write experiments.
 
 ## Regenerating the tutorials