Add some notes on binary

binary.md

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+Binary
+======
+
+> There are only 10 types of people in the world: those who understand binary, and those who don't. -- Unknown
+
+## Converting Decimal to Binary
+
+To convert decimal to binary:
+1. take your decimal number, n
+2. divide your number by 2 and save the dividend and remainder
+3. make the dividend your new number
+4. push the remainder onto a stack
+5. repeat this process until your number is 0
+6. pop all the numbers of the stack to create the binary number
+
+Do this on paper. Stop to understand why the remainder will always be 1 or 0.
+
+FYI: The above technique works with decimal to any base! Just change '2' to the base you want to covert to.
+
+```python
+# Returns a binary string
+# you would never do this in a real program as Python has `bin(n)`
+def decimal2binary(n):
+  nba = [] # n as binary array
+  div = n # dividend
+  while div != 0:
+      rem = div % 2
+      div = div // 2
+      nba.append(rem)
+  # list comprehension to reverse array and join elements as a string
+  return ''.join(str(i) for i in reversed(nba))
+```
+
+## REFERENCES
+
+[Day 10: Binary Numbers](https://www.hackerrank.com/challenges/30-binary-numbers/tutorial). 30 Days of Code. HackerRank.
+