readme: add short introduction #4010

Job	Run time
Compile sysdeps (dripos)	2m 17s
Build mlibc (x86_64, mlibc, gcc)	3m 42s
Compile sysdeps (lemon)	2m 42s
Build mlibc (x86_64, mlibc-static, gcc)	3m 52s
Compile sysdeps (aero)	2m 20s
Build mlibc (x86_64, mlibc-shared, gcc)	3m 35s
Compile sysdeps (ironclad)	2m 16s
Build mlibc (x86_64, mlibc-ansi-only, gcc)	2m 57s
Compile sysdeps (lyre)	2m 18s
Build mlibc (x86_64, mlibc-headers-only, gcc)	1m 59s
Compile sysdeps (keyronex)	2m 23s
Build mlibc (riscv64, mlibc, gcc)	3m 52s
Compile sysdeps (managarm)	2m 35s
Build mlibc (riscv64, mlibc-static, gcc)	4m 33s
Compile sysdeps (vinix)	2m 22s
Build mlibc (riscv64, mlibc-shared, gcc)	3m 21s
Compile sysdeps (astral)	2m 15s
Build mlibc (riscv64, mlibc-ansi-only, gcc)	2m 51s
Build mlibc (riscv64, mlibc-headers-only, gcc)	2m 5s
Build mlibc (aarch64, mlibc, gcc)	4m 0s
Build mlibc (aarch64, mlibc-static, gcc)	3m 16s
Build mlibc (aarch64, mlibc-shared, gcc)	3m 17s
Build mlibc (aarch64, mlibc-ansi-only, gcc)	3m 29s
Build mlibc (aarch64, mlibc-headers-only, gcc)	1m 56s
Build mlibc (x86, mlibc, gcc)	4m 0s
Build mlibc (x86, mlibc-static, gcc)	3m 16s
Build mlibc (x86, mlibc-shared, gcc)	3m 6s
Build mlibc (x86, mlibc-ansi-only, gcc)	3m 25s
Build mlibc (x86, mlibc-headers-only, gcc)	1m 59s
Build mlibc (x86_64, mlibc, clang)	3m 7s
	1h 29m 6s

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