Restore valgrind suppressions file

contrib/valgrind-julia.supp was removed in JuliaLang#38970, but it is still referenced
[in the Valgrind devdocs](https://docs.julialang.org/en/v1/devdocs/valgrind/).
Either the file should be restored, or the reference should be removed
from the documentation.

Here, I have chosen the option of restoring the file, as I think it
provides some value.  I have also updated it such that it does indeed
suppress the warning (JuliaLang#4533) on the latest master.

contrib/README.md

@@ -23,3 +23,4 @@ Debugging
 | Name | Description |
 | ------------------------------ | ----------------------------------------------------------- |
 |[ debug_bootstrap.gdb ](https://github.com/JuliaLang/julia/blob/master/contrib/debug_bootstrap.gdb) | Bootstrap process using the debug build |
+|[ valgrind-julia.supp ](https://github.com/JuliaLang/julia/blob/master/contrib/valgrind-julia.supp) | Suppressions for Valgrind debugging tool |

contrib/valgrind-julia.supp

@@ -0,0 +1,10 @@
+# https://github.com/JuliaLang/julia/issues/4533
+{
+ msync unwind
+ Memcheck:Param
+ write(buf)
+ ...
+ fun:validate_mem
+ ...
+ fun:rec_backtrace
+}