Add explanation of how to enable `maths` feature #1056

Job	Run time
Check documentation	57s
Check file formatting and style	1m 0s
Fuzz Tests	1m 4s
Dependency Tests (stable)	2m 58s
Check minimum rust version	39s
Dependency Tests (beta)	3m 16s
Database Tests - MySQL (stable)	1m 17s
Database Tests - PostgreSQL (stable)	2m 47s
Database Tests - MySQL (beta)	1m 20s
Database Tests - PostgreSQL (beta)	2m 57s
Standard Tests (stable)	2m 18s
Standard Tests (beta)	2m 24s
Database Tests	1s
Test	1s
Pass	1s
	23m 0s

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