Merge pull request #6441 from jorisvandenbossche/doc-sphinxext-readme

DOC: add explanation to doc/sphinxext
COMPAT: infer_freq compat on passing an Index of strings (GH6463)
Merge pull request #6465 from jreback/infer_freq2

COMPAT: infer_freq compat on passing an Index of strings (GH6463)
BUG: split should respect maxsplit when no pat is given
Merge pull request #6467 from cpcloud/str-pat-n-fix-6466

BUG: split should respect maxsplit when no pat is given
DOC: release notes for #6466
Merge pull request #6422 from jseabold/make-missing

ENH: Add functions for creating dataframes with NaNs
ENH: Allow custom frequencies. Closes #4541.
Merge pull request #6473 from jseabold/allow-freq-test-utils

ENH: Allow custom frequencies
Modified get_data_famafrench(name) to allow for all file
extensions. Fixes error when getting "F-F_Momentum_Factor" data (uses
a .TXT extension instead of .txt)

Added a release note in doc/source/releast.rst referencing 6470

Changed the github reference to 6460
BUG: Bug in sum of a timedelta64[ns] series (GH6462)
Merge pull request #6475 from jreback/td

BUG: Bug in sum of a timedelta64[ns] series (GH6462)
BUG: Bug in resample with a timezone and certain offsets (GH6397)
Merge pull request #6477 from jreback/resample

BUG: Bug in resample with a timezone and certain offsets (GH6397)
CLN: remove vestigial count code
Merge pull request #6480 from dsm054/remove_vestigial_count

CLN: remove vestigial count code
PERF: perf improvements in DataFrame construction with a non-daily datelike index (GH6479)

Dynamic vbenches
BUG: fix non-caching of some frequency offsets for date generation

DOC: release notes
DOC: Expand on usage.
Merge pull request #6489 from jseabold/docs-clarification

DOC: Expand on freq in shift usage.