Add openSUSE perl package hints to README.md.

Signed-off-by: Ralf Doering <ralf@rdoering.net>
Don't overwrite AVRDUDE_CONF if AVR_TOOLS_DIR is not set.

If one likes to use avrdude from outside of Arduino together with
Arduino-bundled gcc crosstolchain, the setting of AVRDUDE_CONF got
overwritten.
Mention AVRDUDE and AVRDUDE_CONF in README.md.
Merge pull request #90 from ralfdoering/opensuse

Documention and fixes for openSUSE and system provided avrdude