docs/devel: add some notes on tcg-icount for developers

docs/devel: add some notes on tcg-icount for developers

This attempts to bring together my understanding of the requirements
for icount behaviour into one reference document for our developer
notes.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Pavel Dovgalyuk <dovgaluk@ispras.ru>
Cc: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200709141327.14631-3-alex.bennee@linaro.org>