Add installation instructions to README.md

[OGKevin]

Steps to reproduce:

1. Take a look at https://github.com/bunq/sdk_csharp/blob/8a1a026cc2e9e0e402c54cdc33233f96a3ede004/README.md
2. Installation instructions says TBA.

What should happen:

1. The readme gives an explanation on how to install this package.

What happens:

1. It says TBA

Logs

• Logs no logs

Extra info:

• Tested on 0.12.4