fs-verity: add a documentation file

Add a documentation file for fs-verity, covering:

- Introduction
- Use cases
- User API
    - FS_IOC_ENABLE_VERITY
    - FS_IOC_MEASURE_VERITY
    - FS_IOC_GETFLAGS
- Accessing verity files
- File measurement computation
    - Merkle tree
    - fs-verity descriptor
- Built-in signature verification
- Filesystem support
    - ext4
    - f2fs
- Implementation details
    - Verifying data
        - Pagecache
        - Block device based filesystems
- Userspace utility
- Tests
- FAQ

Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>