Prevent GitHub from excluding the build/ directory (#786)

Add a `.gitattributes` file, [as described on the GitHub documentation](https://docs.github.com/en/search-github/searching-on-github/finding-files-on-github#customizing-excluded-files), to prevent GitHub from excluding this folder from "Go to file". In our repo, this file contains lots of build scripts, so it helps when they're easily findable.

.gitattributes

@@ -0,0 +1,3 @@
+# Prevent GitHub from excluding the build/ directory from the file finder.
+# See: https://docs.github.com/en/search-github/searching-on-github/finding-files-on-github
+build/** linguist-generated=false