docs: Add milestone for 1000 project commits to README

[matthewfeickert]

Description

Add 1000 project commits (with PR #1710) to the list of project milestones in the README. 🎉

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Add milestone to README for 1000 project commits.
   - 1000th commit came from Alexander Held in PR #1710.

[codecov]

Codecov Report

Merging #1739 (531d9e0) into master (7432aba) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1739   +/-   ##
=======================================
  Coverage   98.12%   98.12%           
=======================================
  Files          64       64           
  Lines        4270     4270           
  Branches      683      683           
=======================================
  Hits         4190     4190           
  Misses         46       46           
  Partials       34       34           

Flag	Coverage Δ
contrib	26.25% <ø> (ø)
doctest	60.58% <ø> (ø)
unittests	96.18% <ø> (ø)

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