Add usage note on metrics

[NickCrews]

From: https://academic.oup.com/ije/article/46/5/1699/4107250

...and to derive standard measures of linkage quality (sensitivity/recall, false match rate and positive predictive value/precision).28 These metrics are thought to be more useful than specificity (due to the imbalance between the number of non-matches versus matches in a linkage) or the F-measure (due to issues when comparing linked datasets of different sizes).

[NickCrews]

Also: https://www.valires.com/post/common-pitfalls-to-avoid-when-estimating-er-performance-metrics