add readmes for all packages

[stephenplusplus]

Fixes #1491

Probably easiest to review by looking here: https://github.com/stephenplusplus/gcloud-node/tree/spp--module-readmes/packages

[callmehiphop]

I like it, my only real concern is the duplicated documentation, mainly auth. I understand why we want it here, but it also might be a pain for us to update in the future if it changes. WDYT about linking to auth on the docs site or main README? Or else trying to generate the READMEs programmatically?

[stephenplusplus]

Linking is okay. Let me know if this is overthinking it: if I link to a guide, the URL couldn't be tied to any particular version, so it would go to the latest release, which could be incorrect.

[callmehiphop]

That's a fair point, maybe we should just have lengthy READMEs and if it becomes increasingly difficult to manage, we can rethink it?

[stephenplusplus]

I'm cool with that.

[stephenplusplus]

@callmehiphop PTAL. I updated the "Enable the following APIs" section of each module's readme to include the relevant APIs for the module. Also, I made google-cloud's publish script copy master/readme.md to master/packages/google-cloud/readme.md -- which means all readme updates should be made on master/readme.md.

[coveralls]

Coverage remained the same at 100.0% when pulling c9014c2 on stephenplusplus:spp--module-readmes into d606b2a on GoogleCloudPlatform:master.

[callmehiphop]

@stephenplusplus it looks like there are a ton of references to the old repo/docs (e.g. gcloud-node). Even though there are redirects, we might want to update these.

[stephenplusplus]

Think I caught everything. Can you take a look?

[coveralls]

Coverage remained the same at 99.966% when pulling 6031e69 on stephenplusplus:spp--module-readmes into 77c83a1 on GoogleCloudPlatform:master.