DOC: Fix dead link in README

[AnsonTran]

PR summary

The contribute link in the project's README.md currently links to:
https://matplotlib.org/devdocs/devel/contributing.html

However, this page no longer exists. This PR changes the link to:
https://matplotlib.org/devdocs/devel/contribute.html

PR checklist

• [N/A] "closes #0000" is in the body of the PR description to link the related issue
• [N/A] new and changed code is tested
• [N/A] Plotting related features are demonstrated in an example
• [N/A] New Features and API Changes are noted with a directive and release note
• [N/A] Documentation complies with general and docstring guidelines

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[tacaswell]

There should be a re-direct there:

matplotlib/doc/devel/contribute.rst

Lines 1 to 3 in 478dc5a

	.. redirect-from:: /devel/contributing
	.. _contributing:

We should merge this to avoid the redirect in our links, but we should also sort out why the redirect is not working.

[tacaswell]

I'm going to go ahead and merge this and short-circuit the azure and appveyor CI.

[tacaswell]

Thank you @AnsonTran and congratulations on your first merged PR to Matplotlib 🎉

I hope we hear from you again.

[AnsonTran]

Thanks @tacaswell !