Add docs for UnusedName warning (#411)

* Add docs for `UnusedName` warning

* Update example

errors/UnusedName.md

@@ -0,0 +1,37 @@
+# `UnusedName` Warning
+
+## Example
+
+```purescript
+module UnusedNameExample where
+
+plus :: Int -> Int -> Int
+plus x y = y -- Name x was introduced but not used.
+
+ignore :: forall a. a -> Unit
+ignore value = unit -- Name value was introduced but not used.
+```
+
+## Cause
+
+This warning occurs when a name is introduced but is not used anywhere.
+
+PureScript warns in this case because it could indicate a bug due to a value not being referenced where it should be.
+
+## Fix
+
+If the unused name was unintentional, make use of it to fix the warning:
+
+```purescript
+plus :: Int -> Int -> Int
+plus x y = x + y
+```
+
+If the unused name is intentional, mark the name as unused by prefixing it with a `_`:
+
+```purescript
+ignore :: forall a. a -> Unit
+ignore _value = unit
+```
+
+## Notes