Merge pull request #548 from AFLplusplus/pre-3

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+# AFL Context Sensitive Branch Coverage
+
+## What is this?
+
+This is an LLVM-based implementation of the context sensitive branch coverage.
+
+Basically every function gets its own ID and that ID is combined with the
+edges of the called functions.
+
+So if both function A and function B call a function C, the coverage
+collected in C will be different.
+
+In math the coverage is collected as follows:
+`map[current_location_ID ^ previous_location_ID >> 1 ^ previous_callee_ID] += 1`
+
+## Usage
+
+Set the `AFL_LLVM_INSTRUMENT=CTX` or `AFL_LLVM_CTX=1` environment variable.
+
+It is highly recommended to increase the MAP_SIZE_POW2 definition in
+config.h to at least 18 and maybe up to 20 for this as otherwise too
+many map collisions occur.