docs: add deprecation warning

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  Test
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macos_highsierra
test-macos

00:17:32

 
  Build
failed #630884
build-latest

00:05:09

passed #630885
build-nightly

00:41:45

 
  Docs
skipped #630886
docs
 
  Deploy
skipped #630887
pages
 
Name Stage Failure
failed
build-latest Build
Elapsed: -

An HTTP error occurred when trying to retrieve this URL.
HTTP errors are often intermittent, and a simple retry will get you on your way.
'https://conda.anaconda.org/conda-forge/noarch'


The command '/bin/sh -c conda env create -f /tmp/environment-py27.yml' returned a non-zero code: 1
ERROR: Job failed: exit code 1