Create README.md

Author: Maciej Kot

README.md

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+This program demonstrates a silent signal stack overflow.
+
+What it does:
+
+Mmap anon memory region mem1 and fill it with '1'.
+Spawn a new thread (triggering mmap of a new signal stack).
+Mmap another anon memory region mem2 and fill it with '2'.
+
+Raise a signal. In the signal handler a local array is created and filled with '7'.
+There is a good chance that either mem1 or mem2 gets overwritten by that array.
+We print the last value in both mem1 and mem2 before and after singal handler execution.
+
+Expected behavior 
+  - segfault in the signal handler.
+  
+Observed behavior 
+  - on MacOS 10.15.3 mem1 gets partially overwritten (last val1 changes from 1 to 7)
+  - on Linux mem2 gets partially overwritten (last val2 changes from 2 to 7)
+  - program ends sucessfully (no segfault)
+