Add a warning about cmake in main directory

Add a note to warn that using cmake in the main
project directory will overwrite version controlled
files and this could be annoying.

Author: TGWDB

COMPILING.md

@@ -255,6 +255,10 @@ require manual modification of build files.
    to generate IDE projects by supplying the `-G` flag.  Run `cmake -G` for a
    comprehensive list of supported back-ends.
 
+   Note that if you do not specify the `build` directory the current directory
+   will be the build target. This will modify many version controlled files
+   and be messy to fix.
+
    On macOS >10.14, the build will fail unless you explicitly specify
    the full path to the compiler. This issue is being tracked
    [here](https://github.com/diffblue/cbmc/issues/4956). The invocation thus