chore(readme): add usage

README.md

@@ -1,5 +1,58 @@
+[![Build Status](https://travis-ci.org/spider-gazelle/simple_retry.svg?branch=master)](https://travis-ci.org/spider-gazelle/simple_retry)
+
+
 # Simple Retry
 
-[![Build Status](https://travis-ci.org/spider-gazelle/simple_retry.svg?branch=master)](https://travis-ci.org/spider-gazelle/simple_retry)
+A library for managing blocks of code that you might want to retry when an error occurs.
+
+
+## Installation
+
+1. Add the dependency to your `shard.yml`:
+
+   ```yaml
+   dependencies:
+     simple_retry:
+       github: spider-gazelle/simple_retry
+   ```
+
+2. Run `shards install`
+
+
+## Usage
+
+```crystal
+require "simple_retry"
+
+SimpleRetry.try_to do
+  # Something that should succeed eventually
+  num = rand(10)
+  raise "bad number #{num}" if num < 5
+
+  # Return the number
+  num
+end
+```
+
+There are a number of options you can use to customise
+
+```crystal
+require "simple_retry"
 
-A small library for managing blocks of code that you might want to retry when an error occurs.
+SimpleRetry.try_to(
+  # Runs the block at most 5 times
+  max_attempts: 5,
+  # Will always stop retrying on these errors
+  raise_on: DivisionByZeroError | ArgumentError,
+  # Will only retry on these errors
+  retry_on: Exception,
+  # Initial delay time after first retry
+  base_interval: 10.milliseconds,
+  # Exponentially increase delay up to this period
+  max_interval: 10.seconds,
+  # Adjust the exponential growth by a random amount
+  randomise: 10.milliseconds
+) do |run_count : UInt64, last_error : Exception?, next_delay_time : Time::Span|
+  # Something that should succeed
+end
+```