Bump version to 1.6.0 and update readme

I also needed to change package.json because Jest was giving me the
following error during `safePublish`:

"TypeError: Jest: a transform must export a `process` function."

I fixed this by specifying specific versions of ts-jest and typescript
in testpack's list of things to install; by default the latest versions
are installed, but those are incompatible.

Author: qwertie

package-lock.json

@@ -1,6 +1,6 @@
 {
   "name": "sorted-btree",
-  "version": "1.5.0",
+  "version": "1.6.0",
   "description": "A sorted list of key-value pairs in a fast, typed in-memory B+ tree with a powerful API.",
   "main": "b+tree.js",
   "typings": "b+tree",
@@ -88,8 +88,8 @@
       }
     },
     "install": [
-      "ts-jest",
-      "typescript"
+      "ts-jest@26.4.3",
+      "typescript@3.8.3"
     ],
     "verbose": true,
     "test-folder": ".testpack",

package.json

@@ -377,8 +377,9 @@ Version history
 
 ### v1.6.0 ###
 
-- Added optional second parameter `reusedArray` to `nextHigherPair` and `nextLowerPair` 
-- Optimized 
+- Added `BTree.getPairOrNextLower` and `BTree.getPairOrNextHigher` methods (PR #23)
+- Added optional second parameter `reusedArray` to `nextHigherPair` and `nextLowerPair` (PR #23)
+- Optimizations added in `diffAgainst` (PR #24) and `nextLowerPair` (PR #23)
 
 ### v1.5.0 ###