docs: provide info in readme that plenary will be archived in Q2 2026 #1559

Job	Run time
unit tests (ubuntu-22.04, v0.10.4/nvim-linux-x86_64.tar.gz)	24s
stylua	10s
unit tests (ubuntu-22.04, v0.9.5/nvim-linux64.tar.gz)	18s
unit tests (ubuntu-22.04, v0.8.3/nvim-linux64.tar.gz)	19s
unit tests (ubuntu-22.04, nightly/nvim-linux-x86_64.tar.gz)	22s
Luacheck	23s
	1m 56s

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