README.md: Add information about TSL2561
README.md: Add information about TSL2561

Step by step instructions how to enable i2c and
to use the light sensor TSL2561 through the
example written in the C programming language.

Signed-off-by: Leon Anavi <be92910ae7bb6f58bb9b7c6f845bc30640119592@anavi.org>

leon-anavi/rpi-examples,leon-anavi/rpi-examples,leon-anavi/rpi-examples,leon-anavi/rpi-examples