Acknowledge Purism SPC for providing Librem Key devices

Purism has provided Librem Key hardware devices which allowed the
addition of support for them to the program. With this change we
acknowledge that action with a paragraph in the respective section of
the README.

README.md

@@ -163,7 +163,11 @@ Robin Krahl ([@robinkrahl](https://github.com/robinkrahl)) has been
 a crucial help for the development of **nitrocli**.
 
 The [Nitrokey GmbH][nitrokey-gmbh] has generously provided the necessary
-hardware for developing and testing the program.
+hardware in the form of Nitrokey Pro and Nitrokey Storage devices for
+developing and testing the program.
+
+[Purism][purism] was kind enough to help development of support for
+Librem Keys by providing the necessary hardware devices to test on.
 
 
 License
@@ -190,5 +194,6 @@ the full text of the license.
 [gplv3-tldr]: https://tldrlegal.com/license/gnu-general-public-license-v3-(gpl-3)
 [libnitrokey#32]: https://github.com/Nitrokey/libnitrokey/issues/32
 [libnitrokey#137]: https://github.com/Nitrokey/libnitrokey/issues/137
+[purism]: https://puri.sm/
 [reuse]: https://reuse.software/practices/3.0/
 [semver]: https://semver.org