Create README.md

Author: Nicholas Scheel

README.md

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+# purescript-textcursor
+An isomorphic `TextCursor` type for selections in DOM elements (`<input>` and `<textarea>`). Provides shared DOM methods and TextCursor manipulations.
+
+## Supported elements
+All `<textarea>` elements and `<input>` elements with the `type` attribute set to one of the following:
+- input (default)
+- email
+- search
+- url
+
+## TextCursor
+A `TextCursor` is defined as the following type:
+```purescript
+newtype TextCursor = TextCursor
+    { before :: String
+    , selected :: String
+    , after :: String
+    }
+```
+
+It represents a selection in an element in three regions: the text `before` the cursor/selection, the text inside the `selection` (empty if there is just a cursor), and the text `after`.
+
+What this does is allow transformations to occur (such as replacements, white-space simplification) while preserving the relative location of the cursor or selection.
+
+Note that certain transformations will depend on the position of the text cursor or selection within the field, particularly if they rely on a string of characters versus single characters (e.g. a replacement ` ``|'' -> "`, which depends on quote characters being adjacent, as opposed to `[A-Z] -> [a-z]`, which does not care where the cursor falls as it selects a single character without context).