Add explanation of how to enable `maths` feature #1048

Job	Run time
Fuzz Tests	1m 20s
Check documentation	1m 1s
Dependency Tests (stable)	3m 7s
Check file formatting and style	16s
Check minimum rust version	1m 8s
Dependency Tests (beta)	3m 0s
Database Tests - MySQL (stable)	1m 26s
Database Tests - MySQL (beta)	1m 17s
Database Tests - PostgreSQL (stable)	2m 35s
Standard Tests (stable)	2m 20s
Database Tests - PostgreSQL (beta)	2m 36s
Standard Tests (beta)	2m 16s
Database Tests	1s
Test	1s
Pass	0s
	22m 24s

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