Add readme and license file

Signed-off-by: p4u <pau@dabax.net>

LICENSE

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+Copyright (c) 2018, Respective Authors all rights reserved.
+
+The MIT License
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.

README.md

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+# asmt
+
+asmt is a GoLang sparse merkle tree. 
+
+A forked of [Aergo SMT](https://github.com/aergoio/aergo/tree/develop/pkg/trie) with minor style code modifications 
+and some new methods: Walk() WalkWithRoot() and GetWithRoot().
+
+This module includes an abstraction layer to make the SMT more developer friendly with a very easy to use API:
+
++ Init(name, dataDir string) error
++ LastAccess() (timestamp int64)
++ Add(key, value []byte) error
++ AddBatch(keys, values [][]byte) (failedIndexes []int, err error)
++ GenProof(key, value []byte) (mproof []byte, err error)
++ CheckProof(key, value, root, mproof []byte) (included bool, err error)
++ Root() []byte
++ HashExists(hash []byte) (bool, error)
++ Dump(root []byte) (data []byte, err error) // Dump must return a format compatible with ImportDump
++ DumpPlain(root []byte) (keys [][]byte, values [][]byte, err error)
++ ImportDump(data []byte) error
++ Size(root []byte) (int64, error)
++ Snapshot(root []byte) (Tree, error) // Snapshot is a frozen trie on a specific Root hash that cannot be modified
+