PYTHON-5766 Add codecov badge to readme #3659

Job	Run time
Build wheels for win_amd6	3m 55s
Make SDist	51s
Build wheels for manylinux_x86_64	2m 18s
Build wheels for manylinux_s390x	3m 59s
Build wheels for win_arm64	3m 59s
Build wheels for manylinux_aarch64	3m 59s
Build wheels for manylinux_i686	2m 21s
Build wheels for manylinux_ppc64le	3m 58s
Build wheels for macos	3m 43s
Build wheels for win32	3m 53s
Download Wheels	1s
	32m 57s

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