Add info on how to checkout earlier versions pre-GitHub migrations

It would be great if Git Bootcamp has information on how to checkout git tags to get the earlier CPython releases (e.g. 3.3, 3.4 )

3.3. and 3.4 aren't on CPython's GitHub as branches (unlike 3.7, 3.8, etc), but there are git tags, can can be checked out as follows:

# list all git tags matching v.3.3
git tag -l 'v3.3*'

# checkout the v.3.3.0 tag to a local branch
git checkout tags/v.3.3.0 -b my-own-3.3.0-branch

先在 devguide 中找到 Git Bootcamp 文件，並檢視現有的 checkout CPython 版本指南。加入一段簡短說明，介紹如何使用 tags 取得 3.3 和 3.4 等較早的 release，其中包括 issue 中列出的 tag 清單與 checkout 範例。當讀者能夠找到並依照說明從較早的 tag 建立本機 branch 時，即完成此任務。