Add mailmap (#2)

### Motivation

The contributors list is generated using `git shortlog`.   Git can use the `.mailmap` file to link commits from the same contributor using different email addresses.

### Modifications

Add .mailmap file with initial mappings.

### Result

Contributors using different email addresses will not be duplicated.

### Test Plan

Re-ran soundness script, no change in contributors file.

.mailmap

@@ -0,0 +1 @@
+Euan Harris <[email protected]> <[email protected]>