move nsupdate.info/SPF Toolbox from awesome-selfhosted/DNS to DNS - Control Panels & Domain Management (#504)

- ref. https://github.com/awesome-foss/awesome-sysadmin/pull/473
- ref. https://github.com/awesome-selfhosted/awesome-selfhosted/pull/3938

README.md

@@ -365,9 +365,11 @@ _Related: [DNS - Servers](#dns---servers)_
 - [Designate](https://wiki.openstack.org/wiki/Designate) - DNSaaS services for OpenStack. ([Source Code](https://opendev.org/openstack/designate))
 - [DNSControl](https://stackexchange.github.io/dnscontrol/) - Synchronize your DNS to multiple providers from a simple DSL. ([Source Code](https://github.com/StackExchange/dnscontrol)) `MIT` `Go/Docker`
 - [DomainMOD](https://domainmod.org) - Manage your domains and other internet assets in a central location. ([Source Code](https://github.com/domainmod/domainmod.git)) `GPL-3.0` `PHP`
+- [nsupdate.info](https://www.nsupdate.info/) - Dynamic DNS service. ([Demo](https://www.nsupdate.info/account/register/), [Source Code](https://github.com/nsupdate-info/nsupdate.info)) `BSD-3-Clause` `Python`
 - [octoDNS](https://github.com/github/octodns) - DNS as code - Tools for managing DNS across multiple providers. `MIT` `Python`
 - [Pi-hole](https://pi-hole.net/) - A blackhole for Internet qdvertisements with a GUI for management and monitoring. ([Source Code](https://github.com/pi-hole/pi-hole)) `EUPL-1.2` `Shell/PHP`
 - [Poweradmin](https://www.poweradmin.org/) - Web-based DNS control panel for PowerDNS server. ([Source Code](https://github.com/poweradmin/poweradmin)) `GPL-3.0` `PHP`
+- [SPF Toolbox](https://spftoolbox.com) - Application to look up DNS records such as SPF, MX, Whois, and more. ([Source Code](https://github.com/charlesabarnes/SPFtoolbox)) `MIT` `PHP`
 
 
 ### Editors