Document a critical gotcha in FATE

FATE samples paths can not be contain a '~' character. This has bitten me
on multiple occasions and I have heard I'm not the only victim.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/fate.txt

@@ -12,6 +12,8 @@ Use the following command to get the fate test samples
 To inform the build system about the testsuite location, pass
 `--samples=<path to the samples>` to configure or set the SAMPLES Make
 variable or the FATE_SAMPLES environment variable to a suitable value.
+NOTE: Do not put a '~' character in the samples path to indicate a home
+directory. Because of shell nuances, this will cause FATE to fail.
 
 For information on how to set up FATE to send results to the official FFmpeg
 testing framework, please refer to the following wiki page: