Add README for vfdecrypt

tools/vfdecrypt/README.md

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+# VFDecrypt
+
+[VFDecrypt](https://www.theiphonewiki.com/wiki/VFDecrypt) is used to decrypt Apple iOS root filesystem images. Root filesystem images inside IPSW files are encrypted for iOS >= 9. Since iOS 10, root filesystem images are not encrypted and VFDecrypt is no longer required.
+
+Decrypting root filesystem images requires the decryption key. This is provided for most root filesystem images version by the community at [The iPhone Wiki](https://www.theiphonewiki.com/wiki/Firmware_Keys).
+
+There are several VFDecrypt builds available on the Internet. We've used and updated the implementation [here](https://github.com/trailofbits/iverify-oss/tree/master/vendor/vfdecrypt). We've updated `vfdecrypt.c` to support OpenSSL >= 1.1.0 and the `Makefile` to be OS-dependent.
+
+You can build and run `lzssdec` on macOS and on Linux.
+
+You build `vfdecrypt` using
+
+```
+make
+```
+
+You runn `vfdecrypt` by passing it the root filesystem key, the root filesystem image and the output (decrypted) file:
+
+```
+./vfdecrypt -i ~/Projects/store/out/iPhone5,1_9.3_13E237/058-25512-331.dmg -k 2a66fd6377af8f60d5e300ac3aa8d9c44a1c0dee94579ad3f8a26515debbf381bb971ae8 -o decrypted.dmg
+```
+
+iExtractor runs `vfdecrypt` as part of the `bin/decrypt_fs` and `scripts/decrypt_fs` scripts.