docs: update contributor guidelines

Customize CONTRIBUTING.md intro for GCM and update issue link to point to
the 'New Issue' page rather than the pre-existing issues page.

Author: ldennington

CONTRIBUTING.md

@@ -1,13 +1,13 @@
 # Contributing
 
-[issue]: https://github.com/GitCredentialManager/git-credential-manager/issues
+[issue]: https://github.com/GitCredentialManager/git-credential-manager/issues/new/choose
 [fork]: https://github.com/GitCredentialManager/git-credential-manager/fork
 [pr]: https://github.com/GitCredentialManager/git-credential-manager/compare
 [code-of-conduct]: CODE_OF_CONDUCT.md
 
-Hi there! We're thrilled that you'd like to contribute to this project. Your help is essential for keeping it great.
+Hi there! We're thrilled that you'd like to contribute to GCM :tada:. Your help is essential for keeping it great.
 
-Contributions to this project are [released](https://help.github.com/articles/github-terms-of-service/#6-contributions-under-repository-license) to the public under the [project's open source license](LICENSE).
+Contributions to GCM are [released](https://help.github.com/articles/github-terms-of-service/#6-contributions-under-repository-license) to the public under the [project's open source license](LICENSE).
 
 Please note that this project is released with a [Contributor Code of Conduct][code-of-conduct]. By participating in this project you agree to abide by its terms.