Set repo to normalise linefeeds for text files to lf on checkin

Should avoid mixed linefeeds in the repo. But uses local convention in
checkouts. Details in:
https://www.aleksandrhovhannisyan.com/blog/crlf-vs-lf-normalizing-line-endings-in-git/#all-line-ending-transformations-concern-the-index

.gitattributes

@@ -0,0 +1,9 @@
+#Try to avoid mixed linefeeds in our repo
+# Set the default behavior, in case people don't have core.autocrlf set.
+# (check out and in automatically according to OS)
+* text=auto
+
+# Explicitly declare text files you want to always be normalized and converted
+# to native line endings on checkout.
+#*.html text
+#*.htm text