[SYCL][Doc] Add documentation to use iostream_proxy.hpp instead of <i…

…ostream> (intel#6786)

This is to complement this [PR](intel#6469)
which removes \<iostream\> headers to comply with LLVM Coding Standards.

Signed-off-by: Rauf, Rana <rana.rauf@intel.com>

Signed-off-by: Rauf, Rana <rana.rauf@intel.com>

sycl/doc/developer/ContributeToDPCPP.md

@@ -31,6 +31,14 @@ For any DPC++-related commit, the `[SYCL]` tag should be present in the
 commit message title. To a reasonable extent, additional tags can be used
 to signify the component changed, e.g.: `[PI]`, `[CUDA]`, `[Doc]`.
 
+## Using \<iostream\> 
+According to [LLVM Coding Standards](https://llvm.org/docs/CodingStandards.html#include-iostream-is-forbidden),
+the use `#include <iostream>` is forbidden in library files. Instead, the
+sycl/detail/iostream_proxy.hpp header offers the functionality of <iostream>
+without its static constructor.
+This header should be used in place of <iostream> in DPC++ headers 
+and runtime library files.
+
 ## Tests development
 
 Every product change should be accompanied with corresponding test modification