Update repo to ensure use of commonly searched phrases for DNN extentions

[david-poindexter]

For greater discoverability, I recommend the following:

• Use DNN (formerly DotNetNuke) in the repo Description instead of just DNN.
• Add, at minimum, the topics dnn, dnncms and dotnetnuke just below the repo Description.
• I'll submit a PR shortly to update the README file to add relevant keywords.

[w8tcha]

done