Add IANA reserved IPv6 addresses of 2001 space

[differentreality]

According to https://www.iana.org/assignments/ipv6-address-space/ipv6-address-space.xml Notes 9 through 13, the following IPv6 addresses should be marked as reserved:

• 2001:2::
• 2001:3::
• 2001:4:112::
• 2001:10::
• 2001:20::

[whitequark]

Thanks!