add license, readme, and small to-do list

README.md

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+# string-trie
+
+This repository implements a set over strings in Standard ML using a trie/prefix tree.
+
+The signature provided is:
+
+```
+signature STRING_SET =
+sig
+  (* the type of tries *)
+  type t
+
+  (* the empty trie *)
+  val empty: t
+
+  (* returns true if the trie is empty *)
+  val isEmpty: t -> bool
+
+  (* creates a trie containing just a string *)
+  val fromString: string -> t
+
+  (* returns true if the key was inserted into the trie *)
+  val exists: string * t -> bool
+
+  (* inserts a new string into the trie, returning a new trie *)
+  val insert: string * t -> t
+
+  (* removes the key from the trie, returning a new trie *)
+  val remove: string * t -> t
+
+  (* returns a list of all keys matching the specified prefix *)
+  val getPrefixList: string * t -> string list
+
+  (* returns a list containing all keys in the trie *)
+  val toList: t -> string list
+
+  (* returns a trie containing all keys in the string list *)
+  val fromList: string list -> t
+end
+```
+
+The reason for implementing a new trie specialised to strings rather than using Chris Okasaki's IntMap data structure is to enable prefix searching, where it is possible to get a list of all keys matching a certain prefix.
+
+# To-do
+
+- [ ] Add `foldl`, `foldr`, `foldlWithPrefix`, `foldrWithPrefix` functions to string set
+- [ ] Benchmarks (possibly comparing to a set of strings in a balanced binary tree)
+- [ ] Use unrolled linked list with zipper in trie, to limit size of vector as allocating large vectors repeatedly is expensive
+- [ ] Implement StringMap, containing both keys and values
+
+# Credits
+
+The tests in `tests/string-set-tests.sml` were ported from [kpol's Trie data structure in C3](https://github.com/kpol/trie), although this is not true for the files in the `src/` directory.