forgot readme for ngram
ADD
llvm_mode/README.ngram.md
+# AFL N-Gram Branch Coverage
+
+## Source
+
+This is an LLVM-based implementation of the n-gram branch coverage proposed in
+the paper ["Be Sensitive and Collaborative: Analzying Impact of Coverage Metrics
+in Greybox Fuzzing"](https://www.usenix.org/system/files/raid2019-wang-jinghan.pdf),
+by Jinghan Wang, et. al.
+
+Note that the original implementation (available
+[here](https://github.com/bitsecurerlab/afl-sensitive))
+is built on top of AFL's QEMU mode.
+This is essentially a port that uses LLVM vectorized instructions to achieve
+the same results when compiling source code.
+
+## Usage
+
+The size of `n` (i.e., the number of branches to remember) is an option
+that is specified in the `AFL_LLVM_NGRAM_SIZE` environment variable.
+Good values are 2, 4 or 8.

forgot readme for ngram