Rearrange sources (#4997)

include/xrpl/basics/README.md

@@ -0,0 +1,40 @@
+# Basics
+
+Utility functions and classes.
+
+ripple/basic should contain no dependencies on other modules.
+
+
+Choosing a rippled container.
+=============================
+
+* `std::vector`
+  * For ordered containers with most insertions or erases at the end.
+
+* `std::deque`
+  * For ordered containers with most insertions or erases at the start or end.
+
+* `std::list`
+  * For ordered containers with inserts and erases to the middle.
+  * For containers with iterators stable over insert and erase.
+  * Generally slower and bigger than `std::vector` or `std::deque` except for
+    those cases.
+
+* `std::set`
+  * For sorted containers.
+
+* `ripple::hash_set`
+  * Where inserts and contains need to be O(1).
+  * For "small" sets, `std::set` might be faster and smaller.
+
+* `ripple::hardened_hash_set`
+  * For data sets where the key could be manipulated by an attacker
+    in an attempt to mount an algorithmic complexity attack:  see
+    http://en.wikipedia.org/wiki/Algorithmic_complexity_attack
+
+
+The following container is deprecated
+
+* `std::unordered_set`
+ * Use `ripple::hash_set` instead, which uses a better hashing algorithm.
+ * Or use `ripple::hardened_hash_set` to prevent algorithmic complexity attacks.