readme: add short introduction #4012

Job	Run time
Build mlibc (x86_64, mlibc, gcc)	3m 47s
Build mlibc (x86_64, mlibc-static, gcc)	3m 21s
Build mlibc (x86_64, mlibc-shared, gcc)	3m 13s
Build mlibc (x86_64, mlibc-ansi-only, gcc)	2m 59s
Build mlibc (x86_64, mlibc-headers-only, gcc)	2m 3s
Build mlibc (riscv64, mlibc, gcc)	4m 5s
Build mlibc (riscv64, mlibc-static, gcc)	4m 34s
Build mlibc (riscv64, mlibc-shared, gcc)	3m 5s
Build mlibc (riscv64, mlibc-ansi-only, gcc)	3m 8s
Build mlibc (riscv64, mlibc-headers-only, gcc)	2m 10s
Build mlibc (aarch64, mlibc, gcc)	4m 18s
Build mlibc (aarch64, mlibc-static, gcc)	3m 41s
Compile sysdeps (dripos)	2m 20s
Build mlibc (aarch64, mlibc-shared, gcc)	3m 12s
Build mlibc (aarch64, mlibc-ansi-only, gcc)	3m 4s
Compile sysdeps (lemon)	2m 42s
Build mlibc (aarch64, mlibc-headers-only, gcc)	1m 58s
Build mlibc (x86, mlibc, gcc)	3m 54s
Build mlibc (x86, mlibc-static, gcc)	3m 16s
Compile sysdeps (aero)	2m 18s
Build mlibc (x86, mlibc-shared, gcc)	3m 17s
Compile sysdeps (ironclad)	3m 40s
Build mlibc (x86, mlibc-ansi-only, gcc)	3m 28s
Compile sysdeps (lyre)	2m 15s
Build mlibc (x86, mlibc-headers-only, gcc)	1m 59s
Compile sysdeps (keyronex)	2m 15s
Build mlibc (x86_64, mlibc, clang)	3m 6s
Compile sysdeps (managarm)	2m 35s
Compile sysdeps (vinix)	2m 19s
Compile sysdeps (astral)	2m 21s
	1h 30m 23s

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