Update usage instructions

[eureka-cpu]

The usage instructions in the README alone didn't work for me, I had to use direnv to get it to work last time but it's been a month since I used it and I forgot exactly how to do it 😅

I think previously I did the following, however it no longer works for me (maybe I did it in the wrong order?):

nix-shell -p direnv
direnv allow .envrc.local
eval "$(direnv export bash)"

Then I could use the exporter, but now I just get this:

$ harvest-exporter --year 2024 --month 12 --currency USD --client "client" --format json
[]%  
$ harvest-exporter --month 1 --currency USD --client "client" --format json             
[]%

[eureka-cpu]

If I figure it out before anyone gets to this I'll submit a pull request