hi, all

According to the suggestiongs of some contributors, we have updated “the Guide of Contributor”。
Please Review this Link:https://gitee.com/openeuler/community/pulls/265
The Updates contains:
[1].The operation of PR/Issue has has more detail by picture,example etc.
[2].The new document of "Prepare Development Enviroment " and "Package Complie/Development/Download" is added.
[3].The catalogue of README has been rearranged so that it is clearer and more complete.

The pictures of md need links to pose, so it can not be shown in PR.

Thank you for your Review, wo need your opinions to build a more convenient and open community.