docs: add update one-liner to English README + update.md (#77)

- English README: added update one-liner in hero section and Quick Start
- update.md: added English version of the one-liner for humans

Co-authored-by: Panniantong <panniantong@users.noreply.github.com>

docs/README_en.md

@@ -39,6 +39,12 @@ Install Agent Reach: https://raw.githubusercontent.com/Panniantong/agent-reach/m
 
 Copy that to your Agent. A few minutes later, it can read tweets, search Reddit, and watch Bilibili.
 
+**Already installed? Update in one command:**
+
+```
+Update Agent Reach: https://raw.githubusercontent.com/Panniantong/agent-reach/main/docs/update.md
+```
+
 ### ✅ Before you start, you might want to know
 
 | | |
@@ -83,6 +89,11 @@ Install Agent Reach: https://raw.githubusercontent.com/Panniantong/agent-reach/m
 
 The Agent auto-installs, detects your environment, and tells you what's ready.
 
+> 🔄 **Already installed?** Update in one command:
+> ```
+> Update Agent Reach: https://raw.githubusercontent.com/Panniantong/agent-reach/main/docs/update.md
+> ```
+
 <details>
 <summary>Manual install</summary>