mdhelp: try to describe (briefly) what a network tuner is

docs/markdown/before_begin.md

@@ -10,7 +10,9 @@ television signal and extract from it the programme stream. The tuner hardware
 is also responsible for communicating with your satellite dish via the LNB 
 in the case of DVB-S.
 
-TODO: Network tuners like SAT>IP, HDHomeRun.
+* **Network tuners** are small (usually ([arm](https://en.wikipedia.org/wiki/ARM_architecture) based) 
+computers that you connect to your network via Ethernet or Wifi, they often have a large number of tuners 
+and are controlled via a web interface or software. Many work out-of-the-box with Tvheadend using SAT>IP or the HDHomeRun protocols.
 
 * A **driver** is the piece of software that your operating system uses to
 talk to the tuner. This can be built into the OS (e.g. 'supported since kernel X')