Update README, add gitignore for common CMake and CLion files

.gitignore

+cmake-build-debug/
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README.md

-# at90usb1286-avr-gcc-project
-A CMake project for avr gcc projects specifically setup to work with the at90usb1286 chip.
-Designed for Computer Systems 2 students at the University of Southampton
+# AT90USB1286 AVR-GCC CMake
+A CMake project template for AVR-GCC with the AT90USB1286 chip, primarily useful with CLion and MSYS2 on Windows.
 
-See just below the license header in CMakeLists.txt for brief setup information
+See https://github.com/Gamerick1029/at90usb1286-avr-gcc-project for the project this was forked from, and https://secure.ecs.soton.ac.uk/student/wiki/w/COMP2215_AVR_Tool_Chain_Installation/Windows#CMake_with_MSYS2_.28for_CLion.29 for the guide this relates to.