update README

README.md

@@ -28,12 +28,12 @@ After doing `git pull`, the development may fail to compile because of outdated
 dependencies.  To fix that, please run `opam update` followed by
 `make build-dep`.
 
-## Building Instructions
+## Building
 
 Run `make -jN` to build the full development, where `N` is the number of your
 CPU cores.
 
-## Structure
+## Directory Structure
 
 * The folder [algebra](theories/algebra) contains the COFE and CMRA
   constructions as well as the solver for recursive domain equations.