docs: extend yarn docs (#647)

## This PR

- add @openfeature/core to yarn install instructions

---------

Signed-off-by: Michael Beemer <[email protected]>
Co-authored-by: Todd Baert <[email protected]>

Author: beeme1mr

packages/client/README.md

@@ -60,7 +60,8 @@ npm install --save @openfeature/web-sdk
 #### yarn
 
 ```sh
-yarn add @openfeature/web-sdk
+# yarn requires manual installation of the @openfeature/core peer-dependency
+yarn add @openfeature/web-sdk @openfeature/core
 ```
 
 ### Usage

packages/server/README.md

@@ -57,7 +57,8 @@ npm install --save @openfeature/server-sdk
 #### yarn
 
 ```sh
-yarn add @openfeature/server-sdk
+# yarn requires manual installation of the @openfeature/core peer-dependency
+yarn add @openfeature/server-sdk @openfeature/core
 ```
 
 ### Usage