modify meta.yml and generate README.md for new building instructions #955

Job	Run time
build (mathcomp/mathcomp:2.1.0-coq-8.16)	16m 23s
build (mathcomp/mathcomp:2.1.0-coq-8.18)	34m 47s
build (mathcomp/mathcomp:2.2.0-coq-8.16)	16m 31s
build (mathcomp/mathcomp:2.2.0-coq-8.19)	16m 5s
build (mathcomp/mathcomp:2.2.0-coq-dev)	16m 41s
build (mathcomp/mathcomp-dev:coq-8.19)	15m 47s
build (mathcomp/mathcomp-dev:coq-dev)	16m 26s
	2h 12m 40s

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