add clarity for custom path installation

install.sysconfdir is another value, in addition to install.prefix,
that could be set for custom path installation.

Signed-off-by: Naveen R. Iyer <iyernaveenr@gmail.com>

INSTALL.md

@@ -80,8 +80,9 @@ See [the rustc-dev-guide for more info][sysllvm].
    ```
 
    If you plan to use `x.py install` to create an installation, it is
-   recommended that you set the `prefix` value in the `[install]` section to a
-   directory: `./configure --set install.prefix=<path>`
+   recommended that you set the `prefix` and `sysconfdir` values in the
+   `[install]` section to a directory: `./configure --set install.prefix=<path>
+   --set install.sysconfdir=<path>
 
 3. Build and install: