Add section README/contents page

Seems like this is necessary for the mdbook
summary to generate correctly.

Author: Nicholas Clarke

src/SUMMARY.md

@@ -1,7 +1,7 @@
 # Summary
 
 - [Introduction](introduction/README.md)
-- Principles
+- [Principles](principles/README.md)
   - [Optimise for the worst case](principles/optimise-worst.md)
 - [Network](network/README.md)
   - [Multiplexing](network/multiplexing.md)

src/principles/README.md

@@ -0,0 +1,5 @@
+# Principles
+
+This section contains general principles to apply when developing Cardano nodes.
+
+1. [Optimise only for the worst case](./optimise-worst.md)