Update release instructions

[dsherry]

I took some more notes when we shipped v0.9.0 and want to get them into the release guide.

For easier review: here's the copy on master, and here's the copy from this PR.

[jeremyliweishih]

Nice - I like these changes!

[codecov]

Codecov Report

Merging #733 into master will increase coverage by 11.48%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #733       +/-   ##
===========================================
+ Coverage   87.81%   99.29%   +11.48%     
===========================================
  Files         140      140               
  Lines        4981     4981               
===========================================
+ Hits         4374     4946      +572     
+ Misses        607       35      -572     

Impacted Files	Coverage Δ
evalml/tests/component_tests/test_components.py	100.00% <0.00%> (+0.60%)	⬆️
evalml/automl/auto_base.py	97.05% <0.00%> (+0.98%)	⬆️
evalml/tests/pipeline_tests/test_pipelines.py	99.63% <0.00%> (+3.26%)	⬆️
evalml/tests/component_tests/test_utils.py	96.42% <0.00%> (+3.57%)	⬆️
evalml/pipelines/pipeline_base.py	100.00% <0.00%> (+5.85%)	⬆️
evalml/tests/utils_tests/test_dependencies.py	100.00% <0.00%> (+6.25%)	⬆️
...ts/automl_tests/test_auto_classification_search.py	100.00% <0.00%> (+15.66%)	⬆️
.../tests/automl_tests/test_auto_regression_search.py	100.00% <0.00%> (+25.77%)	⬆️
evalml/tests/pipeline_tests/test_graphs.py	100.00% <0.00%> (+26.02%)	⬆️
...nents/estimators/classifiers/xgboost_classifier.py	100.00% <0.00%> (+34.28%)	⬆️
... and 13 more

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