documentation: explain where the build output goes.

pch: rebased pullreq #189

README.md

@@ -62,9 +62,9 @@ the instructions in the "Building using make" section.
     This will create a `config.mk` file at the top level of the repository. You
     can set some defaults in `config.mk`
 
- 2. In the sketch directory, type `make` to build for
-    APM2. Alternatively, `make apm1` will build for the APM1 and `make
-    px4` will build for the PX4.
+ 2. In the sketch directory, type `make` to build for APM2. Alternatively,
+    `make apm1` will build for the APM1 and `make px4` will build for the PX4.
+    The binaries will generated in `/tmp/<i>sketchname</i>.build`.
 
  3. Type `make upload` to upload. You may need to set the correct default
     serial port in your `config.mk`.