Update documentation to include new MergeIlLinkerFiles parameter (#60)

Adding support for the new /illink switch, introduced [here](gluck/il-repack@986c1db#diff-3ee934cbb99395b6bc4f805ef4e686e6d80b0b8e1c27d552ed9590eae4f4bb6fR256).

README.md

@@ -109,6 +109,7 @@ You can turn this functionality off by setting ClearOutputDirectory to False as
 | KeyFile                        | Specifies a key file to sign the output assembly.                                                                                                          |
 | LibraryPath                    | List of paths to use as "include directories" when attempting to merge assemblies.                                                                         |
 | LogFile                        | Specifies a log file to output log information.                                                                                                            |
+| MergeIlLinkerFiles             | Merge IL Linker file XML resources from Microsoft assemblies (optional). Same-named XML resources ('ILLink.*.xml') will be combined during merging.        |
 | NoRepackRes                    | Does not add the embedded resource 'ILRepack.List' with all merged assembly names.                                                                         |
 | OutputFile                     | Output name for the merged assembly.                                                                                                                       |
 | Parallel                       | Use as many CPUs as possible to merge the assemblies.                                                                                                      |