README: Suggest bootstrapping out-of-source for development

Building in-source and running the test suite in-source both leave
many artifacts that we (purposely) do not cover via `.gitignore`.
For developing CMake we typically use out-of-source builds, so
suggest this in the README.

README.rst

@@ -57,12 +57,19 @@ You need to have a C++ compiler (supporting C++11) and a ``make`` installed.
 Run the ``bootstrap`` script you find in the source directory of CMake.
 You can use the ``--help`` option to see the supported options.
 You may use the ``--prefix=<install_prefix>`` option to specify a custom
-installation directory for CMake. You can run the ``bootstrap`` script from
-within the CMake source directory or any other build directory of your
-choice. Once this has finished successfully, run ``make`` and
-``make install``.  In summary::
+installation directory for CMake.  Once this has finished successfully,
+run ``make`` and ``make install``.
 
- $ ./bootstrap && make && sudo make install
+For example, if you simply want to build and install CMake from source,
+you can build directly in the source tree::
+
+  $ ./bootstrap && make && sudo make install
+
+Or, if you plan to develop CMake or otherwise run the test suite, create
+a separate build tree::
+
+  $ mkdir cmake-build && cd cmake-build
+  $ ../cmake-source/bootstrap && make
 
 Windows
 ^^^^^^^
@@ -79,9 +86,7 @@ There are two ways for building CMake under Windows:
 
      $ pacman -S --needed git base-devel mingw-w64-x86_64-gcc
 
-   and bootstrap as above::
-
-     $ ./bootstrap && make
+   and bootstrap as above.
 
 .. _`CMake Download Page`: https://cmake.org/cmake/resources/software.html
 .. _`MSYS2`: https://www.msys2.org/