Note on signature mismatch error?

[kngwyu]

I got an error that says:

adb: failed to install backAndForth-debug.apk: Failure [INSTALL_FAILED_UPDATE_INCOMPATIBLE: Package jp.oist.abcvlib.backandforth signatures do not match previously installed version; ignoring!]

when installing an apk. Though uninstalling the previously installed package easily solves this problem, it may be better to note this on README.
Also, maybe we can note in which situation what adb install option is required.

[topherbuckley]

I think for now adding it to the README would be easiest. I can add this signatures and permissions issues to my next scope of work. See linked PR above. Feel free to edit/add to the PR if you want other changes.