chore: discourage AdministratorAccess policy (aws#12196)

Even though we can't actually suggest any particular policy to use
other than `AdministratorAccess`, we've been requested to make it
very clear that developers shouldn't be using `AdministratorAccess`
without thinking (after an engagement in which an organization's
CCoE was displeased with individual developer teams copy/pasting
example bootstrapping commands from our developer guides).

Add a note to tell developers to check with their organization's CCoE
before using `AdministratorAccess`.


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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/pipelines/README.md

@@ -648,6 +648,14 @@ These command lines explained:
 > account only to bootstrap it and provision the initial pipeline. Otherwise,
 > access to administrative credentials should be dropped as soon as possible.
 
+<br>
+
+> **On the use of AdministratorAccess**: The use of the `AdministratorAccess` policy
+> ensures that your pipeline can deploy every type of AWS resource to your account.
+> Make sure you trust all the code and dependencies that make up your CDK app.
+> Check with the appropriate department within your organization to decide on the
+> proper policy to use.
+
 ### Migrating from old bootstrap stack
 
 The bootstrap stack is a CloudFormation stack in your account named