Added documentation and a simple example for the custom mutator functionality

docs/custom_mutator.txt

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+==================================================
+Adding custom mutators to AFL using
+==================================================
+This file describes how you can implement custom mutations to be used in AFL.
+
+Implemented by Khaled Yakdan from Code Intelligence <yakdan@code-intelligence.de>
+
+
+1) Description
+--------------
+
+Custom mutator libraries can be passed to afl-fuzz to perform custom mutations
+on test cases beyond those available in AFL - for example, to enable structure-aware
+fuzzing by using libraries that perform mutations according to a given grammar.
+
+The custom mutator library is passed to afl-fuzz via the AFL_CUSTOM_MUTATOR_LIBRARY
+environment variable. The library must export the afl_custom_mutator() function and
+must be compiled as a shared object. For example:
+     $CC -shared -Wall -O3 <lib-name>.c -o <lib-name>.so
+
+AFL will call the afl_custom_mutator() function every time it needs to mutate
+a test case. For some cases, the format of the mutated data returned from
+the custom mutator is not suitable to directly execute the target with this input.
+For example, when using libprotobuf-mutator, the data returned is in a protobuf
+format which corresponds to a given grammar. In order to execute the target,
+the protobuf data must be converted to the plain-text format expected by the target.
+In such scenarios, the user can define the afl_pre_save_handler() function. This function
+is then transforms the data into the format expected by the API before executing the target.
+afl_pre_save_handler is optional and does not have to be implemented if its functionality
+is not needed.
+
+2) Example
+----------
+A simple example is provided in ../custom_mutators/