add gemspec file

Author: khiga8

erblint-github.gemspec

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+Gem::Specification.new do |s|
+    s.name = "erblint-github"
+    s.version = "0.0.1"
+    s.summary = "erblint GitHub"
+    s.description = "Code style checking for GitHub Ruby repositories"
+    s.homepage = "https://github.com/github/erblint-github"
+    s.license = "MIT"
+  
+    s.files = Dir["README.md", "LICENSE"]
+  
+    s.required_ruby_version = ">= 2.5.0"
+  
+    s.email = ["opensource+erblint-github@github.com"]
+    s.authors = ["GitHub Open Source"]
+  end