nand2tetris chapter 3: add notes.

books/nand2tetris/notes/3_sequential_logic.md
	-Add notes for chapter 3, "Sequential Logic", of nand2tetris.
nand2tetris project 3: implement Bit chip.

books/nand2tetris/projects/3_sequential_logic/Bit.hdl
	-Implement a `Bit` chip.
nand2tetris project 3: implement Register chip.

books/nand2tetris/projects/3_sequential_logic/Register.hdl
	-Implement the `Register` chip.
nand2tetris project 3: implement RAM8 chip.

books/nand2tetris/projects/3_sequential_logic/RAM8.hdl
	-Implement the `RAM8` chip.
nand2tetris project 3: implement RAM64 chip.

books/nand2tetris/projects/3_sequential_logic/RAM64.hdl
	-Implement a `RAM64` chip.
nand2tetris project 3: implement RAM512 chip.

books/nand2tetris/projects/3_sequential_logic/RAM512.hdl
	-Implement a `RAM512` chip.
nand2tetris project 3: implement RAM4K chip

books/nand2tetris/projects/3_sequential_logic/RAM4K.hdl
	-Implement a `RAM4K` chip.
nand2tetris project 3: implement RAM16K chip.

books/nand2tetris/projects/3_sequential_logic/RAM16K.hdl
	-Implement a `RAM16K` chip.
nand2tetris project 3: implement PC chip.

books/nand2tetris/projects/3_sequential_logic/PC.hdl
	-Implement a `PC` chip.
Merge branch 'books/nand2tetris.3_sequential_logic' into books/nand2tetris

books/nand2tetris/
(notes/3_sequential_logic.md, projects/3_sequential_logic/)
	-Complete the 3rd chapter, "Sequential Logic", of nand2tetris.