Add short description to example/ binaries

examples/index.rs

@@ -1,3 +1,4 @@
+/// Benchmark regular indexing flow (using JSONRPC), don't persist the resulting index.
 extern crate electrs;
 extern crate error_chain;
 

examples/load.rs

@@ -1,3 +1,5 @@
+/// Benchmark bulk indexing flow (using local blk*.dat files).
+/// Persist (and compact) the resulting index
 extern crate electrs;
 
 #[macro_use]