README needs improvement

[thibeaux]

• I'm sure there are spelling and grammar errors that I missed that need to be corrected.

• I also need to explain what the struct is doing and what data it represents because the original code doesn't really specify, and I don't want to assume. So I will have to run some test and do some investigating.

• I would also like to add more content to the usage section to explain why and how to use the newly added functions, even though this is sort of documented in the INA219.c file. Also explain the rational for each of the functions.

• I would like to add more detail to the "building the project" section to explain why I added the wire going from the VIN- pin to the external power supply pin of the micro controller. To remove any guessing for anyone interested in the project.