Re-instate principles section, but only as a single page

Author: ch1bo

src/SUMMARY.md

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

src/optimise-worst.md renamed to src/principles/README.md

@@ -1,3 +1,7 @@
+# Principles
+
+This section contains general principles to apply when developing Cardano nodes.
+
 ## Optimise only for the worst case
 
 Algorithms often possess different performance characteristics in the best,