Add note about API key requirement link in Linear toolkit doc (#199)

Author: Ansub

tools/toolkits/linear.mdx

@@ -5,6 +5,8 @@ title: Linear
 **LinearTool** enable an Agent to perform [Linear](https://linear.app/) tasks.
 
 ## Prerequisites
+The following examples require Linear API key, which can be obtained from [here](https://linear.app/settings/account/security).
+
 
 ```shell
 export LINEAR_API_KEY="LINEAR_API_KEY"