Document whether CWD is supposed to be hashed or not.

[t184256]

I've spent several hours figuring out that "Absolute paths to files must match to get a cache hit." doesn't mean CWD is included into a hash. Or at least that's what I think now. Could it be spelled out in the clear, please?

[luser]

See #35 for some historical discussion.

For Rust compilation, the cwd is included:

sccache/src/compiler/rust.rs

Line 1375 in 3f318a8

// 8. The cwd of the compile. This will wind up in the rlib.

#208 made this work somewhat better for C++ compilation.