Add hiring promotion section to README (#1323)

Add a prominent 'We're Hiring!' callout to the README promoting open Engineer and Developer Relations positions at Zep, linking to the careers page.

Author: danielchalef

README.md

@@ -27,6 +27,10 @@ Graphiti
 
 </div>
 
+> [!NOTE]
+> **We're Hiring!** Build context graphs that power reliable, personalized, fast production AI agents.
+> Come build with us — we're hiring Engineers and Developer Relations folks. [View open roles](https://www.getzep.com/careers/).
+
 ⭐ *Help us reach more developers and grow the Graphiti community. Star this repo!*
 
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