[docs] Add hook warning note

[dmtrKovalenko]

No description provided.

[cypress]

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Test summary

50 • 0 • 0 • 0

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Run details

Project	material-ui-pickers
Status	Passed
Commit	b12957f
Started	Jul 31, 2019 1:55 PM
Ended	Jul 31, 2019 1:56 PM
Duration	00:52 💡
OS	Linux Debian - 8.11
Browser	Electron 61

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[codecov]

Codecov Report

Merging #1220 into next will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             next   #1220      +/-   ##
=========================================
+ Coverage   94.13%   94.2%   +0.07%     
=========================================
  Files          56      56              
  Lines        1414    1414              
  Branches      199     199              
=========================================
+ Hits         1331    1332       +1     
+ Misses         65      64       -1     
  Partials       18      18

Impacted Files	Coverage Δ
lib/src/views/Clock/ClockPointer.tsx	96% <0%> (+4%)	⬆️

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