Add license file as a suggestion as GPL-2.0-only

[joelparkerhenderson]

My HubSpot employee technical contact tells me that this HubSpot repo is open for customer use, which is great because GraphQL can be very helpful.

I see that the repo does not have a license, and GitHub is recommending adding a license. So this pull request is to suggest adding a typical license file such as GPL-2.0-only, which is the normal Linux license.

If the HubSpot web team needs a different license, can you change this pull request purpose and add the license of your choice?

The license needs to be in place for me to be able to use the code because of compliance.

Thanks!