mailmap: map Isaac's old email to the new one

I assume that I'm not the only one who has changed/will change their
email over the years of river development, so this seems like a
reasonable thing to have.

 .mailmap | 1 +
 1 file changed, 1 insertion(+)

diff --git a/.mailmap b/.mailmap
new file mode 100644
index 0000000..0e050b3
--- /dev/null
+++ b/.mailmap
@@ -0,0 +1 @@
+Isaac Freund <[email protected]> <[email protected]>