Add gitpod support (#157)

Author: LinuxSuRen

.gitpod.Dockerfile

@@ -0,0 +1,5 @@
+FROM gitpod/workspace-full
+
+RUN sudo rm -rf /usr/bin/hd && \
+    brew install linuxsuren/linuxsuren/hd && \
+    hd install cli/cli

.gitpod.yml

@@ -0,0 +1,13 @@
+image:
+  file: .gitpod.Dockerfile
+
+tasks:
+  - init: |
+      [[ ! -z "${DOCKER_USER}" && ! -z "${DOCKER_PASSWD}" ]] && docker login -u${DOCKER_USER} -p${DOCKER_PASSWD}
+      git config --global user.name $GIT_AUTHOR_NAME
+      git config --global user.email $GIT_COMMITTER_EMAIL
+      gh repo fork --remote
+
+vscode:
+  extensions:
+    - redhat.java

README.md

@@ -1,3 +1,4 @@
+[![Gitpod ready-to-code](https://img.shields.io/badge/Gitpod-ready--to--code-blue?logo=gitpod)](https://gitpod.io/#https://github.com/jenkinsci/gitlab-branch-source-plugin)
 [![Build Status](https://ci.jenkins.io/job/Plugins/job/gitlab-branch-source-plugin/job/master/badge/icon)](https://ci.jenkins.io/job/Plugins/job/gitlab-branch-source-plugin/job/master/)
 [![Travis](https://img.shields.io/travis/jenkinsci/gitlab-branch-source-plugin.svg?logo=travis&label=build&logoColor=white)](https://travis-ci.org/jenkinsci/gitlab-branch-source-plugin)
 [![GitHub release](https://img.shields.io/github/release/jenkinsci/gitlab-branch-source-plugin.svg?label=release)](https://github.com/jenkinsci/gitlab-branch-source-plugin/releases/latest)