Create README.md

Author: shiffman

167_Prime_Spiral/README.md

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+# Prime Spiral
+
+## Some useful facts:
+
+    - All primes except 2 are `odd`.
+    - All primes `greater` than 3 can be written in the form  `6k - 1` or `6k + 1`.
+
+Using those facts we can optimaze isPrime function to check only **_numbers 6k - 1 and 6k + 1_**
+instead of ~~all the numbers between 1 and the given number~~ .
+
+source :[Project_Euler](https://projecteuler.net/overview=007)
+
+```javascript
+// Function to test if number is prime in P5.js
+function isPrime(value) {
+  if (value < 2) return false;
+  else if (value < 4) return true;
+  else if (value % 2 == 0 || value % 3 == 0) return false;
+  else {
+    for (let i = 5; i <= sqrt(value); i += 6) {
+      if (value % i == 0 || value % (i + 2) == 0) {
+        return false;
+      }
+    }
+  }
+
+  return true;
+}
+```
+
+Thank you to @MoulatiMehdi for the above note!