add docs about crossing dask and stream nodes

[CJ-Wright]

This closes #173 with docs

[CJ-Wright]

@mrocklin @martindurant I think this is ready for review!

[martindurant]

(I think this will need to merge from master to pass)

[codecov-io]

Codecov Report

Merging #176 into master will increase coverage by 0.47%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #176      +/-   ##
==========================================
+ Coverage   93.05%   93.53%   +0.47%     
==========================================
  Files          13       13              
  Lines        1483     1546      +63     
==========================================
+ Hits         1380     1446      +66     
+ Misses        103      100       -3

Impacted Files	Coverage Δ
streamz/dataframe/core.py	92.17% <0%> (ø)	⬆️
streamz/sources.py	95.31% <0%> (+3.08%)	⬆️

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[martindurant]

Please ignore what I said, seems to work fine and, magically, increases coverage. Perhaps the doc is picked up by doctest?

@mrocklin , happy with the words I suggested?

[mrocklin]

Sorry. I'm slammed right now and not planning to check in on this project this week.

…

On Tue, Mar 26, 2019 at 7:25 AM Martin Durant ***@***.***> wrote: Please ignore what I said, seems to work fine and, magically, increases coverage. Perhaps the doc is picked up by doctest? @mrocklin <https://github.com/mrocklin> , happy with the words I suggested? — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#176 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AASszH-r5ELuWFAYUho-3UTuC0T-iDWPks5vai3mgaJpZM4T6Rcx> .

[martindurant]

Understood.

Would you be interested in granting me rights to merge documentation PRs like this and close issues as I see fit? I would not merge real code which changes functionality (I know github doesn't differentiate) without your sayso.