Documentations: Ecosystem/API - pydatastream (issue #5630)
Following an old issue #5630 (Documentation / Remote Data Access - 3rd pary libraries).

Quandl was already described in Ecosystem / API section, so I've added link to pydatastream that targets Thomson Reuters Datastream.

closes #5630.

Does this close  #5630 fully?
 @jorisvandenbossche - OK, fixed
 I think it fully closes #5630.

As a second thought, I'm not sure if it makes sense to add support to Quandl/python or pydatastream directly in web.DataReader. It will amount to 

```
import quandl
return Quandl.get(...)
```

or 

```
from pydatastream import Datastream
DWE = Datastream(username="...", password="...")
return DWE.fetch(...)
```

What do you think?
 I also think including this in pandas is not needed. First you raise the point of the non-free data source. And if the packages have a nice interface to get these data as you show above, they can certainly be a package on their own. We are also thinking of rather moving some of the data readers included in pandas to separate packages, rather than including more of them (see #8961). It is very easy to install eg the quandl package.

Then, for this PR: we always ask to squash the commits into one commit (you can do this with `git rebase -i upstream/master`) for small changes. Can you do this? 
If problems, just ask, or I can also do it on merge.
 Just to be sure: after rebase I'm getting another branch (as the first one was already pushed to github).
Do I need to submit another PR from the new branch?
 Normally, the command I gave does rebase your existing branch (doc_ecosystem_api), so you can just push it again to your fork, and this PR will automatically be updated (you will only have to 'force push': `git push --force origin doc_ecosystem_api`
 Aha, force push! Thanks, I did not know about it

Done then
 OK, thanks!
 And if you are interested more in general in the data readers, certainly chime in on the linked issue #8961 
