Add changelog file (backport modifications?)

Author: mateoconlechuga

CHANGELOG.md

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+## Toolchain
+
+#### Version 7.0
+ + Add scripts to build/clean libraries and examples easily
+ + Move spasm to /bin rather than /lib/src/include
+ + Fix demo 11 readme
+ 
+#### Version 6.95
+ + Modify cstartup.asm to disable usb timers and reset them on exit
+
+## Libraries
+
+#### Version 6.95
+  + `graphx` Optimize gfx_Darken to be about 25% faster
+  + `graphx` Optimize gfx_Lighten to be about 19% faster
+  + `graphx` Characters 0x01-0x1F are considered printable rather than control
+  + `graphx` Add support for width/height of tiles not powers of 2
+  + `graphx` Fix bug with tile index 255