README of translation API is old.

[c-bata]

In which file did you encounter the issue?

https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/translate/cloud-client/README.rst

Did you change the file? If so, how?

Describe the issue

I want to use Google Translate API.
So I read the README and get API Key from CloudPlatform Console.

But, the snippets don't use API Key . So I searched the issues and found following issue.

#659

According to this issue, API Key is not required, but README still haven't updated.
I want to know the usage of Google Translate SDK.

[gguuss]

I updated the README in this PR, changes should be merged, feel free to reopen if the instructions are still causing issues for you.