Add comment about implementing SChip's quirks

src/chip8.cpp

@@ -262,6 +262,11 @@ void Chip8::Exec(const opcode_t& opc) {
     static std::mt19937 seed(std::random_device{}());
     static std::uniform_int_distribution<uint8_t> rng(0, 255);
 
+    // NOTES: The implementation below follows Erik Bryntse's instructions.
+    // (http://devernay.free.fr/hacks/chip8/schip.txt). It does implement
+    // SCHIP-1.x's quirks #5, #6, #11 described here:
+    // https://chip8.gulrak.net
+    // Without them, I found out that several ROMs were crashing.
 #define EXEC_INSTRUCTION \
     /* assembly, condition, instruction(s) */ \
     X("CLS"        , prefix == 0x0 && nnn == 0x0e0 , Cls();) \