add 2.5 notes

notes/2.5_git_checkout.md

@@ -0,0 +1,16 @@
+## 2.5 Git Checkout
+
+### 2.5.1 HEAD reset = checkout
+
+查看当前的branch
+```bash
+git branch -v
+```
+
+checkout到commit的父亲，使得head指针指向父提交，而不是branch
+```bash
+git checkout <commit>^
+```
+
+
+