Make note of symlinks. (#1031)

* Make note of symlinks.

It took me awhile before figuring out I should use hardlinks instead of symlinks. This proposal suggests adding a note in the hope it'll save someone else a bit of time. 

See also this issue: https://github.com/mozilla/sccache/issues/993 where someone else also spent way too much time on this issue.

README.md

@@ -240,3 +240,7 @@ Known Caveats
 * Incrementally compiled crates cannot be cached. By default, in the debug profile Cargo will use incremental compilation for workspace members and path dependencies. [You can disable incremental compilation.](https://doc.rust-lang.org/cargo/reference/profiles.html#incremental)
 
 [More details on Rust caveats](/docs/Rust.md)
+
+### Symbolic links
+
+* Symbolic links to sccache won't work. Use hardlinks: `ln sccache /usr/local/bin/cc`