docs: note that cdk.context.json must be committed (#12565)

Many people don't commit or want to commit `cdk.context.json`, and
are subsequently annoyed that they can't use `Vpc.fromLookup()` in a
pipeline.

Make it very clear that this is the way CDK is designed to work and
they must do it.

Relates to #12160

----

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packages/@aws-cdk/aws-ec2/README.md

@@ -382,6 +382,11 @@ otherwise. Subnet names will be determined from the `aws-cdk:subnet-name` tag
 on the subnet if it exists, or will mirror the subnet type otherwise (i.e.
 a public subnet will have the name `"Public"`).
 
+The result of the `Vpc.fromLookup()` operation will be written to a file
+called `cdk.context.json`. You must commit this file to source control so
+that the lookup values are available in non-privileged environments such
+as CI build steps, and to ensure your template builds are repeatable.
+
 Here's how `Vpc.fromLookup()` can be used:
 
 [importing existing VPCs](test/integ.import-default-vpc.lit.ts)