Update git usage page with advice on pull requests etc

We could update http://biopython.org/wiki/GitUsage to be much shorter (refer to more external resources at GitHub or elsewhere rather than duplicate things) (_update: perhaps not, see below_), and concentrate on for Biopython specific topics like our development model (and link to http://biopython.org/wiki/Contributing or a new page specifically on coding styles, testing, etc).

For example, since we've tried to continue a single-branch model, most of our pull requests have been rebased and fast-forward merged (giving a linear history). To facilitate this, it can be helpful for the contributor to sometimes rebase and update their pull request (`git fetch origin` then ``git rebase origin/master` and `git push your_fork your_branch --force`), although often the person merging the PR will do this at the command line.

從 GitUsage 頁面開始，並將其與 Contributing 頁面和相關的 GitHub 資源進行比較。在留言串中確認預期範圍，然後縮短指南，同時保留 Biopython 特有的開發模型指引，包括 pull-request rebasing 和 fast-forward merging。

由索引模型根據 Issue 內容生成。