chore: Update READMEs to reflect new cross-stack sharing mechanism (#1556)

Author: rix0rrr

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

@@ -37,21 +37,21 @@ const certificate = new Certificate(this, 'Certificate', {
 
 ### Importing
 
-Import a certificate either manually, if you know the ARN:
+Import a certificate manually, if you know the ARN:
 
 ```ts
 const certificate = Certificate.import(this, 'Certificate', {
     certificteArn: "arn:aws:..."
 });
 ```
 
-Or use exporting and importing mechanisms between stacks:
+### Sharing between Stacks
 
-```ts
-const certRef = certStack.certificate.export();
+To share the certificate between stacks in the same CDK application, simply
+pass the `Certificate` object between the stacks.
 
-const certificate = Certificate.import(this, 'Certificate', certRef);
-```
 
-> We should probably also make a Custom Resource that can looks up the certificate ARN
-> by domain name by querying ACM.
+## TODO
+
+- [ ] Custom Resource that can looks up the certificate ARN by domain name by querying ACM.
+- [ ] Custom Resource to automate certificate validation through Route53.

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

@@ -172,13 +172,15 @@ The `VpcNetwork` above will have the exact same subnet definitions as listed
 above. However, this time the VPC will have only 1 NAT Gateway and all
 Application subnets will route to the NAT Gateway.
 
-#### Sharing VPCs across stacks
+#### Sharing VPCs between stacks
 
 If you are creating multiple `Stack`s inside the same CDK application, you
 can reuse a VPC defined in one Stack in another by using `export()` and
 `import()`:
 
-[sharing VPCs between stacks](test/example.share-vpcs.lit.ts)
+[sharing VPCs between stacks](test/integ.share-vpcs.lit.ts)
+
+#### Importing an existing VPC
 
 If your VPC is created outside your CDK app, you can use `importFromContext()`: