Added basic documentation to README.

README

@@ -10,13 +10,34 @@ system, `cargo`. At which point you simply run `cargo build` in the root
 directory of the project. To create an optimized release build append
 the `--release` flag to the previous command.
 
+# Usage
+-------
+The purpose of Indivisible is to find the nth prime and all the primes
+before it. The basic usage is `indivisible <n>` where `n` is the ordinal
+of the prime you'd like to find. To display all primes before `n`, you
+can run verbose mode by using the `--verbose` or simply `-v` option.
+
+Since Indivisible generates primes using previously computed primes, you
+can also import prime numbers previously computed with the `--import` or
+`-i` option. To store already computed primes you are expected to use
+piping like any UNIX user would expect. Here is an example:
+
+  # store first 100 primes in ./primes
+  indivisible -v 100 > ./primes
+  # appends next 400 primes
+  indivisible -i ./primes -v 500 >> ./primes
+  # display the 600th prime
+  indivisible -i ./primes 600
+
 # Legacy
 --------
-This project was originally written in C. It can be found at
+This project was originally written in C. It can be found on my GitLab
-<https://gitlab.com/naortega/Indivisible-legacy>
+page.[1]
 
 # License
 ---------
 This project is licensed under the terms & conditions of the GNU General
 Public License version 3 or greater (see `LICENSE` file for more
 information).
+
+[1] https://gitlab.com/naortega/Indivisible-legacy