update doc

README.md

@@ -34,6 +34,14 @@ with:
   xmake-version: branch@master
 ```
 
+Use specified pr:
+
+```yml
+uses: xmake-io/github-action-setup-xmake@v1
+with:
+  xmake-version: pr@1
+```
+
 Use semver:
 
 ```yml

action.yml

@@ -9,7 +9,7 @@ inputs:
   xmake-version:
     required: true
     default: latest
-    description: The version to use. Should be a semver range or 'latest'
+    description: The version to use. Should be a semver range or 'latest'. Or use pr@{pr_number} to select a pr, use branch@{branch_name} to select a branch
 
 runs:
   using: 'node12'