Add a README

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+# Linux Kernel Patch
+
+There are two pieces, the `zstd_compress` and `zstd_decompress` kernel modules, and the BtrFS patch.
+The patches are based off of the linux kernel version 4.9.
+The BtrFS patch is not present in its entirety yet.
+
+## Zstd Kernel modules
+
+* The header is in `include/linux/zstd.h`.
+* It is split up into `zstd_compress` and `zstd_decompress`, which can be loaded independently.
+* Source files are in `lib/zstd/`.
+* `lib/Kconfig` and `lib/Makefile` need to be modified by applying `lib/Kconfig.diff` and `lib/Makefile.diff` respectively.
+* `test/UserlandTest.cpp` contains tests for the patch in userland by mocking the kernel headers.
+  It can be run with the following commands:
+  ```
+  cd test
+  make googletest
+  make UserlandTest
+  ./UserlandTest
+  ```
+
+## BtrFS
+
+* `fs/btrfs/zstd.c` is provided.
+* Some more glue is required to integrate it with BtrFS, but I haven't included the patches yet.
+  In the meantime see https://github.com/terrelln/linux/commit/1914f7d4ca6c539369c84853eafa4ac104883047 if you're interested.