[Fix] Readme terms tzkt

[jdsika]

As discussed I changed the wording in the readme to make clear that tzkt is hust an option and not mandatory.

• Check list:

[ ] I extended the Travis CI test units with the corresponding tests for this new feature (if needed).

[x] I extended the Sphinx documentation (if needed).

[ ] I extended the help section (if needed).

[x] I changed the README file (if needed).

[ ] I created a new issue if there is further work left to be done (if needed).

Work effort: 0.3h

[utdrmac]

Looks good! Thank you.