Add note about need libev for build examples
(May be also add check if libev is available because the build fail :
In file included from client.cc:38:0:
client.h:38:16: fatal error: ev.h: No such file or directory
@@ -38,6 +38,10 @@ libngtcp2 uses cunit for its unit test frame work:
* cunit >= 2.1
+To build sources under the examples directory, libev is required:
+
+* libev
+
The client and server under examples directory require boringssl as
crypto backend:
