Add android to os list informally. Closes [skip ci]
@@ -121,6 +121,7 @@ These are provided by the `.system()` method call.
| Value | Comment |
| ----- | ------- |
+| android | By convention only, subject to change |
| cygwin | The Cygwin environment for Windows |
| darwin | Either OSX or iOS |
| dragonfly | DragonFly BSD |
