Document requirement for trio to run examples

Author: uSpike

README.rst

@@ -8,6 +8,8 @@ It is based on libgpiod and its CFFI adapter by Steven P. Goldsmith
 <[email protected]>, as downloaded from
 `github <https://github.com/sgjava/userspaceio.git>`_.
 
+To run examples, make sure to install `trio` first.
+
 Testing AsyncGPIO requires a Linux distribution that enables the mock-GPIO module.
 As of mid-2020, Debian's kernel does not include this module, but Raspbian's does.