add readme and modify screenshots

Signed-off-by: zhangxinlogo <zhangxin360@huawei.com>

ability/Fraction/README.md

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+# Fraction<a name="EN-US_TOPIC_0000001172202293"></a>
+
+### Introduction<a name="section103mcpsimp"></a>
+
+This sample provides an ability-based layout: fraction. A fraction has its own lifecycle and can be placed in an ability container. You can perform the following operations on fractions: add, replace, hide, show, push into the stack, and pulled from the stack.
+
+### Usage<a name="section105mcpsimp"></a>
+
+1.  On the home screen of your app, touch  **Replace Fraction**  to replace the fraction.
+2.  Touch  **Next Page**  to go to the next page.
+3.  Touch  **Hide Fraction**  to hide the fraction. The button then changes to  **Show Fraction**, touching which will show the fraction again.
+4.  Touch  **Push into Stack**  to push the fraction into the stack. The button then changes to  **Pop from Stack**, touching which will pull the fraction from the stack.
+
+### Constraints<a name="section111mcpsimp"></a>
+
+This sample can only be run on standard-system devices.
+