moved rrweb-snapshot into packages/rrweb-snapshot

packages/rrweb-snapshot/README.md

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+# rrweb-snapshot
+
+[![Build Status](https://travis-ci.org/rrweb-io/rrweb-snapshot.svg?branch=master)](https://travis-ci.org/rrweb-io/rrweb-snapshot) [![Join the chat at https://gitter.im/rrweb-io/rrweb-snapshot](https://badges.gitter.im/rrweb-io/rrweb-snapshot.svg)](https://gitter.im/rrweb-io/rrweb-snapshot?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+
+Snapshot the DOM into a stateful and serializable data structure.
+Also, provide the ability to rebuild the DOM via snapshot.
+
+## API
+
+This module export following methods:
+
+### snapshot
+
+`snapshot` will traverse the DOM and return a stateful and serializable data structure which can represent the current DOM **view**.
+
+There are several things will be done during snapshot:
+
+1. Inline some DOM states into HTML attributes, e.g, HTMLInputElement's value.
+2. Turn script tags into `noscript` tags to avoid scripts being executed.
+3. Try to inline stylesheets to make sure local stylesheets can be used.
+4. Make relative paths in href, src, CSS to be absolute paths.
+5. Give an id to each Node, and return the id node map when snapshot finished.
+
+#### rebuild
+
+`rebuild` will build the DOM according to the taken snapshot.
+
+There are several things will be done during rebuild:
+
+1. Add data-rrid attribute if the Node is an Element.
+2. Create some extra DOM node like text node to place inline CSS and some states.
+3. Add data-extra-child-index attribute if Node has some extra child DOM.
+
+#### serializeNodeWithId
+
+`serializeNodeWithId` can serialize a node into snapshot format with id.
+
+#### buildNodeWithSN
+
+`buildNodeWithSN` will build DOM from serialized node and store serialized information in `__sn` property.