Clarify symlink handling on Windows

docs/contributing.rst

@@ -94,6 +94,13 @@ dependencies (`evmone <https://github.com/ethereum/evmone/releases>`_,
 On macOS some of the testing scripts expect GNU coreutils to be installed.
 This can be easiest accomplished using Homebrew: ``brew install coreutils``.
 
+On Windows systems make sure that you have a privilege to create symlinks,
+otherwise several tests may fail.
+Administrators should have that privilege, but you may also
+`grant it to other users <https://docs.microsoft.com/en-us/windows/security/threat-protection/security-policy-settings/create-symbolic-links#policy-management>`_
+or
+`enable Developer Mode <https://docs.microsoft.com/en-us/windows/apps/get-started/enable-your-device-for-development>`_.
+
 Running the Tests
 -----------------