README: BASIC control mode does not steer with pedals (Ferret's 510 report)

Field report (new tester, 510): keyboard A/D + arrows 'not working' --
can twist and drive but not turn.  Root-caused live via key injection:
fresh spawns start in BASIC control mode, where steering rides the
STICK (authentic rookie mode) and the pedal-bound keys are inert by
design; one M press (MID) makes A/D steer.  (A false regression-bisect
chase established the metric noise came from an attached pad's stick
drift nudging BASIC steering.)  README now warns about the default
mode; kept the env-gated [padwrite] diagnostic from the hunt.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

engine/MUNGA_L4/L4PADRIO.cpp

@@ -707,6 +707,18 @@ void
 				// centered (inside the deadzone) it leaves the keyboard
 				// integration alone.
 				//
+				// DIAG (BT_PAD_LOG=1): who overwrites which channel?
+				if (getenv("BT_PAD_LOG"))
+				{
+					static int s_padWriteLog = 0;
+					if (s_padWriteLog < 60)
+					{
+						++s_padWriteLog;
+						DEBUG_STREAM << "[padwrite] axis " << binding.axis
+							<< " -> channel " << binding.channel
+							<< " value " << raw << std::endl << std::flush;
+					}
+				}
 				channelValue[binding.channel] = raw;
 			}
 			else if (previousPadAxisRaw[a] != 0.0f)