Add .gitattributes file to 'vendor' mustache templates

Created by: MikeRalphson

PR checklist

• Read the contribution guidelines.
• n/a Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
• n/a Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

This has the effect of making GitHub attribute the language of the project correctly (e.g. as Java). This is useful for sites which list projects by assigned language.