Add custom 404 page

Author: lwasser

404.md

@@ -0,0 +1,17 @@
+---
+layout: splash
+permalink: /404.html
+title: Oops! I couldn't find the page that you want. Page Not Found
+feature_404:
+  - title: "Oopsies"
+    excerpt: "Oopsies"
+---
+
+## Are you looking for any of the following:
+
+* [our package guide](https://www.pyopensci.org/python-package-guide)
+* [our peer review guide](https://www.pyopensci.org/peer-review-guide)
+
+If not, please try to use the top navigation to get to the right spot.
+If you think there is an error on our site - [please open an issue here](https://github.com/pyOpenSci/pyopensci.github.io/issues).
+