Renamed "Beginner" label back to "good first issue" per github default/best practice (#176)

Signed-off-by: Adrian Sutton <adrian.sutton@consensys.net>

CONTRIBUTING.md

@@ -102,9 +102,9 @@ Enhancement suggestions are tracked as [GitHub issues](https://guides.github.com
 * **Specify the name and version of the OS you're using.**
 
 ## Your First Code Contribution
-Start by looking through the 'beginner' and 'help wanted' issues:
-* [Beginner issues][search-label-beginner] - issues which should only require a few lines of code, and a test or two.
-* [Help wanted issues][search-label-help-wanted] - issues which are a bit more involved than `beginner` issues.
+Start by looking through the 'good first issue' and 'help wanted' issues:
+* [Good First Issue][search-label-good-first-issue] - issues which should only require a few lines of code, and a test or two.
+* [Help wanted issues][search-label-help-wanted] - issues which are a bit more involved than `good first issue` issues.
 
 ### Local Development
 The codebase is maintained using the "*contributor workflow*" where everyone without exception contributes patch proposals using "*pull-requests*". This facilitates social contribution, easy testing and peer review.
@@ -177,8 +177,8 @@ Exceptions:
 | `enhancement` | [search][search-label-enhancement] | Feature requests. |
 | `bug` | [search][search-label-bug] | Confirmed bugs or reports that are very likely to be bugs. |
 | `help wanted` | [search][search-label-help-wanted] | The core team would appreciate help from the community in resolving these issues. |
-| `beginner` | [search][search-label-beginner] | Less complex issues which would be good first issues to work on for users who want to contribute. |
-| `info needed` | [search][search-label-more-information-needed] | More information needs to be collected about these problems or feature requests (e.g. steps to reproduce). |
+| `good first issue` | [search][search-label-good-first-issue] | Less complex issues which would be good first issues to work on for users who want to contribute. |
+| `info needed` | [search][search-label-info-needed] | More information needs to be collected about these problems or feature requests (e.g. steps to reproduce). |
 | `needs reproduction` | [search][search-label-needs-reproduction] | Likely bugs, but haven't been reliably reproduced. |
 | `blocked` | [search][search-label-blocked] | Issues blocked on other issues. |
 | `duplicate` | [search][search-label-duplicate] | Issues which are duplicates of other issues, i.e. they have been reported before. |
@@ -216,7 +216,7 @@ Exceptions:
 [search-label-enhancement]: https://github.com/search?q=is%3Aopen+is%3Aissue+repo%3APegasysEng%2FPantheon+label%3Aenhancement
 [search-label-bug]: https://github.com/search?q=is%3Aopen+is%3Aissue+repo%3APegasysEng%2FPantheon+label%3Abug
 [search-label-help-wanted]: https://github.com/search?q=is%3Aopen+is%3Aissue+repo%3APegasysEng%2FPantheon+label%3Ahelp%20wanted
-[search-label-beginner]: https://github.com/search?q=is%3Aopen+is%3Aissue+repo%3APegasysEng%2FPantheon+label%3Abeginner
+[search-label-good-first-issue]: https://github.com/search?q=is%3Aopen+is%3Aissue+repo%3APegasysEng%2FPantheon+label%3Agood%20first%20issue
 [search-label-info-needed]: https://github.com/search?q=is%3Aopen+is%3Aissue+repo%3APegasysEng%2FPantheon+label%3Ainfo%20needed
 [search-label-needs-reproduction]: https://github.com/search?q=is%3Aopen+is%3Aissue+repo%3APegasysEng%2FPantheon+label%3Aneeds%20reproduction
 [search-label-blocked]: https://github.com/search?q=is%3Aopen+is%3Aissue+repo%3APegasysEng%2FPantheon+label%3Ablocked