pkg/adt: README initial commit

Signed-off-by: Gyuho Lee <leegyuho@amazon.com>

pkg/adt/README.md

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+
+## Red-Black Tree
+
+*"Introduction to Algorithms" (Cormen et al, 3rd ed.), Chapter 13*
+
+1. Every node is either red or black.
+2. The root is black.
+3. Every leaf (NIL) is black.
+4. If a node is red, then both its children are black.
+5. For each node, all simple paths from the node to descendant leaves contain the
+same number of black nodes.
+
+For example,
+
+```go
+import (
+    "fmt"
+
+    "go.etcd.io/etcd/pkg/adt"
+)
+
+func main() {
+    ivt := adt.NewIntervalTree()
+    ivt.Insert(NewInt64Interval(510, 511), 0)
+    ivt.Insert(NewInt64Interval(82, 83), 0)
+    ivt.Insert(NewInt64Interval(830, 831), 0)
+    ...
+```
+
+After inserting the values `510`, `82`, `830`, `11`, `383`, `647`, `899`, `261`, `410`, `514`, `815`, `888`, `972`, `238`, `292`, `953`.
+
+![red-black-tree-01-insertion.png](img/red-black-tree-01-insertion.png)
+
+Deleting the node `514` should not trigger any rebalancing:
+
+![red-black-tree-02-delete-514.png](img/red-black-tree-02-delete-514.png)
+
+Deleting the node `11` (TODO)
+
+Try yourself at https://www.cs.usfca.edu/~galles/visualization/RedBlack.html.