Update Lambda README to include more examples

[soda0289]

This commit adds examples for modifying the response headers, to enable CORS, and how to read the event and context variables using an options function.

The lack of documentation has been brought up in the original PR and in this issue:
#315

TODO:

• Update CHANGELOG.md with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

[stubailo]

Thanks!

[kkpoon]

my implementation

set CORS_ORIGINS in lambda environment variable

CORS_ORIGINS="http://localhost:8080,http://example.com,https://example.com"

var corsOrigins = process.env.CORS_ORIGINS
// ......
server.graphqlLambda({
    //...
})(event, context, function(err, output) {
    var matchedCORS = corsOrigins
        .split(",")
        .map(function(o) { return o.trim(); })
        .filter(function(o) { return o === event.headers.origin; });
    if (matchedCORS.length > 0) {
        output.headers = output.headers || {};
        output.headers['Access-Control-Allow-Credentials'] = 'true';
        output.headers['Access-Control-Allow-Origin'] = event.headers.origin;
    }
});

[soda0289]

I like this example and will use it in my project. It might be a little complex for the README but it does show how to validate domain names instead of just allowing everything. I think I will add one more example that checks the origin of the request. Thanks.

[DxCx]

LGTM in general, but im less familiar with aws,
@NeoReyad can you give a short review as well?

[soda0289]

@NeoReyad is just my work account, I use this account more often, but wrote the integration for work. I will probably have to add some more debugging information later on since several questions still come up.

[DxCx]

oh cool 👍
can you open an issue with the open questions so people can take a look and review / respond?
maybe the experience of the others might help you as well ;)