Modify Web .config using powershell

Hi folks,

                  while changing the site to claims based authentication we need to make some entry in the web.config of the sharepoint site,as a part of that we need to add  the following entry  under configuration/system.web/membership/providers and set thedefaultProvider =LDAPProvider

        <addname="LDAPProvider"type="xxxxxxxxxxxxxxxxxxxxxx, Microsoft.SharePoint, Version=14.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" />

how can we achieve this through the power shell?We are doing the Auto Deployment through the power shell

Thanks & Regards

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