Update readme

[moonorblue]

I add some essential import modules and update constants in README, to make the sample code up-to-date with this wrapper :)

[jaggedsoft]

Thank you for your contribution. Looks good to me, though we might need to specify how to get open orders instead of just trade history.

[jaggedsoft]

instead if import Decimal I had to do this

from decimal import *

[moonorblue]

import Decimal

It was a typo in my previous commit, I think from decimal import Decimal should work :)

[moonorblue]

Thank you for your contribution. Looks good to me, though we might need to specify how to get open orders instead of just trade history.

I think the original READEME should be clear with the example open_orders = client.get_open_orders('tbnb185tqzq3j6y7yep85lncaz9qeectjxqe5054cgn') ,
And trade history should be clear in the comment I updated in commit e0c93 ?