chore(docs): add documents of db (#888)

Author: cool-develope

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### Improvements
 
+- [#874](https://github.com/cosmos/iavl/pull/874) Decouple `cosmos-db` and implement own `db` package.
 - [#695](https://github.com/cosmos/iavl/pull/695) Add API `SaveChangeSet` to save the changeset as a new version.
 - [#703](https://github.com/cosmos/iavl/pull/703) New APIs `NewCompressExporter`/`NewCompressImporter` to support more compact snapshot format.
 - [#729](https://github.com/cosmos/iavl/pull/729) Speedup Genesis writes for IAVL, by writing in small batches.

db/README.md

@@ -0,0 +1,28 @@
+# DB
+
+The `db` package contains the key-value database interface and `memdb` implementation. The `memdb` is a simple in-memory key-value store that is used for testing and development purposes.
+
+## Context
+
+The main purpose of the `db` package is to provide decoupling between `cosmos-db` and `iavl` packages. It provides a simple `wrapper` for the old users of `iavl` and `cosmos-db` to use the new `db` package without changing their code. For example:
+
+```go
+package main
+
+import (
+    "cosmossdk.io/log"
+	dbm "github.com/cosmos/cosmos-db"
+
+	"github.com/cosmos/iavl"
+	idbm "github.com/cosmos/iavl/db"
+)
+
+func main() {
+    levelDB, err := dbm.NewDB("application", dbm.GoLevelDBBackend, "test")
+	if err != nil {
+		panic(err)
+	}
+
+	tree := iavl.NewMutableTree(idbm.NewWrapper(dbm.NewPrefixDB(levelDB, []byte("s/k:main/"))), 0, false, log.NewNopLogger())
+}
+```