qasan readme

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 This library is the injected runtime used by QEMU AddressSanitizer (QASan).
 
+The original repository is [here](https://github.com/andreafioraldi/qasan).
+
+The version embedded in qemuafl is an updated version of just the usermode part and this runtime in injected via LD_PRELOAD (so works just for dynamically linked binaries).
+
+The usage is super simple, just set the env var `AFL_USE_QASAN=1` when fuzzing in qemu mode (-Q). afl-fuzz will automatically set AFL_PRELOAD to load this library and enable the QASan instrumentation in afl-qemu-trace.
+
+For debugging purposes, we still suggest to run the original QASan as the stacktrace support for ARM (just a debug feature, it does not affect the bug finding capabilities during fuzzing) is WIP.
+
+### When I should use QASan?
+
+If your target binary is PIC x86_64, you should before give a try to [retrowrite](https://github.com/HexHive/retrowrite) for static rewriting.
+
+If it fails, or if your binary is for another architecture, or you want to use persistent and snapshot mdoe, AFL++ QASan mode is what you want/have to use.
+
+Note that the overhead of libdislocator when combined with QEMU mode is much lower but it can catch less bugs. This is a short blanket, take your choice.