Update changelog and improve docs on skipped test

[skip ci]

Author: Byron

doc/source/changes.rst

@@ -2,6 +2,11 @@
 Changelog
 =========
 
+2.1.6 - Bugfixes
+====================================
+
+* support for worktrees
+
 2.1.3 - Bugfixes
 ====================================
 

git/test/test_submodule.py

@@ -664,11 +664,12 @@ def test_add_empty_repo(self, rwdir):
         # end for each checkout mode
 
     @skipIf(HIDE_WINDOWS_KNOWN_ERRORS,
-            """FIXME: ile "C:\\projects\\gitpython\\git\\cmd.py", line 671, in execute
+            """FIXME on cygwin: File "C:\\projects\\gitpython\\git\\cmd.py", line 671, in execute
                 raise GitCommandError(command, status, stderr_value, stdout_value)
             GitCommandError: Cmd('git') failed due to: exit code(128)
               cmdline: git add 1__Xava verbXXten 1_test _myfile 1_test_other_file 1_XXava-----verbXXten
               stderr: 'fatal: pathspec '"1__çava verböten"' did not match any files'
+             FIXME on appveyor: see https://ci.appveyor.com/project/Byron/gitpython/build/1.0.185
                 """)
     @with_rw_directory
     def test_git_submodules_and_add_sm_with_new_commit(self, rwdir):