document cmake clean per #87

INSTALL.md

@@ -136,6 +136,15 @@ ccmake ..
 Going forward, we plan to focus on conda as our preferred environment, and
 eventually plan to support direct installation via conda.
 
+If you want to clean out the build generated by cmake, cd into the build directory and 
+say 
+```
+cmake --build . --target clean
+```
+
+If you updated an existing git repo from loos 3.x, we suggest running `scons -c; scons -c config` 
+before building loos.
+
 ## Where's my stuff?
 
 Installing into the conda distribution behaves somewhat differently from a