Add .gitignore.

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..24956f6
--- /dev/null
+++ b/.gitignore

@@ -0,0 +1,16 @@
+# Ignore backup files.
+*~
+# Ignore Vim swap files.
+.*.swp
+# Ignore files generated by IDEs.
+/.classpath
+/.factorypath
+/.idea/
+/.ijwb/
+/.project
+/.settings
+# Ignore all bazel-* symlinks. There is no full list since this can change
+# based on the name of the directory bazel is cloned into.
+/bazel-*
+# Ignore outputs generated during Bazel bootstrapping.
+/output/