Add documentation on throttling

[alexgolec]

Lots of interesting tribal knowledge to be shared:

• 120 per second seems to be enforced pretty strictly
• Throttling happens on a per-account basis, which means you can't game it by setting up multiple API keys

[pssolanki111]

from what I've been seen so far, one API key can be used with multiple accounts. (i think someone in the server is actually doing it? haven't done personally). but as per the docs too, the limits are on accounts which implies that different tokens for different accounts should not be counted under same limit even though they were generated using same API.

While this doesn't make a lot of sense for someone having just one account, it is extremely useful for people writing web apps. They can create one developer account and thus it saves their end users from having to create developer accounts, setting redirect url and stuff.

Also as Zuko pointed out, having more than one token.json file for one account and same API key or one account but different API keys would get throttle limit all on the tokens combined as they refer to the same tda account.

None of it is personally tested as i don't have an account with tda.