引入三方flutter_markdown

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lib/components/flutter_markdown/README.md

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+# Flutter Markdown
+[![pub package](https://img.shields.io/pub/v/flutter_markdown.svg)](https://pub.dartlang.org/packages/flutter_markdown) 
+[![Build Status](https://travis-ci.org/flutter/flutter_markdown.svg?branch=master)](https://travis-ci.org/flutter/flutter_markdown)
+
+
+A markdown renderer for Flutter. It supports the
+[original format](https://daringfireball.net/projects/markdown/), but no inline
+html.
+
+## Getting Started
+
+Using the Markdown widget is simple, just pass in the source markdown as a
+string:
+
+    new Markdown(data: markdownSource);
+
+If you do not want the padding or scrolling behavior, use the MarkdownBody
+instead:
+
+    new MarkdownBody(data: markdownSource);
+
+By default, Markdown uses the formatting from the current material design theme,
+but it's possible to create your own custom styling. Use the MarkdownStyle class
+to pass in your own style. If you don't want to use Markdown outside of material
+design, use the MarkdownRaw class.
+
+## Image support
+
+The `Img` tag only supports the following image locations:
+
+* From the network: Use a URL prefixed by either `http://` or `https://`.
+
+* From local files on the device: Use an absolute path to the file, for example by
+  concatenating the file name with the path returned by a known storage location,
+  such as those provided by the [`path_provider`](https://pub.dartlang.org/packages/path_provider)
+  plugin.
+
+* From image locations referring to bundled assets: Use an asset name prefixed by `resource:`.
+  like `resource:assets/image.png`.