Initialize the Semiprimes Service

This is an exciting take-home assignment because I get to write a service in
Elixir!

assessments/semiprimes/README.md

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+# Semiprimes Service
+
+## Introduction
+
+A **composite** is a number containing at least two prime factors. For example:
+
+```
+15 = 3 × 5
+9 = 3 × 3
+12 = 2 × 2 × 3
+```
+
+There are ten composites below thirty containing precisely two, not necessarily
+distinct, prime factors: `4, 6, 9, 10, 14, 15, 21, 22, 25, 26`. Let’s call such
+numbers *Semiprimes*.
+
+## Task
+
+- Write a module which provides a function to tell whether a given number, `N`,
+  is a semiprime. `N` will be 10^5.
+- Please implement an API (RESTful or GraphQL) to factor a given number into two
+  prime numbers if it’s a semiprime, otherwise, return an error message.
+
+## Stretch Goals
+
+- Handle the invalid inputs.
+- Support batch requests: i.e. users could provide 100 numbers, and the API
+  return the answer for all.
+- Considering this module will be used by a long running service, could you
+  optimize it to give answers faster?