config-linux: Explicitly allow symlinks for providing devices

I'd rather address runtime compliance by breaking this down into
explicit checks based on POSIX stat(3) calls.  But with that approach
rejected [1], mentioning symlinks here helps motivate runtime-tools'
choice of os.Stat [2,3] (which follows symlinks) vs. os.Lstat (which
does not [4]).

[1]: #829 (comment)
[2]: https://github.com/opencontainers/runtime-tools/blob/f5c82b3918bdfc3ed4b594dcfab4d1554beaf992/cmd/runtimetest/main.go#L319
[3]: https://golang.org/pkg/os/#Stat
[4]: https://golang.org/pkg/os/#Lstat

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

config-linux.md

@@ -110,7 +110,7 @@ Note that the number of mapping entries MAY be limited by the [kernel][user-name
 ## <a name="configLinuxDevices" />Devices
 
 **`devices`** (array of objects, OPTIONAL) lists devices that MUST be available in the container.
-The runtime MAY supply them however it likes (with [`mknod`][mknod.2], by bind mounting from the runtime mount namespace, etc.).
+The runtime MAY supply them however it likes (with [`mknod`][mknod.2], by bind mounting from the runtime mount namespace, using symlinks, etc.).
 
 Each entry has the following structure: