README.md in urlcrypt-0.1.2 vs README.md in urlcrypt-0.2.0

@@ -18,11 +18,11 @@
 
   * Loading a generated image from your web app in an email
   * Links that come with an expiration date (à la S3)
   * Mini-apps that don't persist data on the server
 
-Works with Ruby 2.1+
+Works with Ruby 2.6+
 
 **Important**: As a general guideline, URL lengths shouldn't exceed about 2000
 characters in length, as URLs longer than that will not work in some browsers
 and with some (proxy) servers. This limits the amount of data you can store
 with URLcrypt.
@@ -39,22 +39,25 @@
 Add to your Gemfile:
 ```ruby
 gem 'urlcrypt', '~> 0.1.1', require: 'URLcrypt'
 ```
 
+Then, set `ENV['urlcrypt_key']` to the default encryption key you will be using. This should be at least a 256-bit AES key, see below. **To ensure your strings are encoded, URLcrypt uses `ENV.fetch` to check for the variable, so it _must_ be set.**
+
 ## Example
 
 ```ruby
-# encrypt and encode with 256-bit AES
-# one-time setup, set this to a securely random key with at least 256 bits, see below
-URLcrypt.key = '...'
-
-# now encrypt and decrypt!
+# encrypt and decrypt using the default key from ENV['urlcrypt_key']!
 URLcrypt.encrypt('chunky bacon!')        # => "sgmt40kbmnh1663nvwknxk5l0mZ6Av2ndhgw80rkypnp17xmmg5hy"
 URLcrypt.decrypt('sgmt40kbmnh1663nvwknxk5l0mZ6Av2ndhgw80rkypnp17xmmg5hy')
   # => "chunky bacon!"
 
+# If needed, you can specify a custom key per-call
+URLcrypt.encrypt('chunky bacon!', key: '...')        # => "....."
+URLcrypt.decrypt('sgmt40kbmnh1663nvwknxk5l0mZ6Av2ndhgw80rkypnp17xmmg5hy', key: '...')
+# => "chunky bacon!"
+
 # encoding without encryption (don't use for anything sensitive!), doesn't need key set
 URLcrypt.encode('chunky bacon!')          # => "mnAhk6tlp2qg2yldn8xcc"
 URLcrypt.decode('mnAhk6tlp2qg2yldn8xcc')  # => "chunky bacon!"
 ```
 
@@ -68,10 +71,10 @@
 ```
 
 To use the key with URLcrypt, you'll need to convert that from a hex string into a real byte array:
 
 ```ruby
-URLcrypt::key = ['longhexkeygoeshere'].pack('H*')
+ENV['urlcrypt_key'] = ['longhexkeygoeshere'].pack('H*')
 ```
 
 ## Running the Test Suite
 
 If you want to run the automated tests for URLcrypt, issue this command from the