Add .gitignore file

This commit imports .gitignore from rubocop-hq/rubocop.
https://github.com/rubocop-hq/rubocop/blob/v0.58.1/.gitignore

.gitignore

@@ -0,0 +1,61 @@
+# rcov generated
+coverage
+coverage.data
+
+# rdoc generated
+rdoc
+
+# yard generated
+doc
+.yardoc
+
+# bundler
+.bundle
+Gemfile.lock
+Gemfile.local
+
+# rbenv
+.ruby-version
+
+# rspec
+/spec/examples.txt
+/.test_queue_stats
+
+# jeweler generated
+pkg
+
+# etags
+TAGS
+
+# Have editor/IDE/OS specific files you need to ignore? Consider using a global gitignore:
+#
+# * Create a file at ~/.gitignore
+# * Include files you want ignored
+# * Run: git config --global core.excludesfile ~/.gitignore
+#
+# After doing this, these files will be ignored in all your git projects,
+# saving you from having to 'pollute' every project you touch with them
+#
+# Not sure what to needs to be ignored for particular editors/OSes? Here's some ideas to get you started. (Remember, remove the leading # of the line)
+#
+# For MacOS:
+#
+#.DS_Store
+
+# For TextMate
+#*.tmproj
+#tmtags
+
+# For emacs:
+#*~
+#\#*
+#.\#*
+
+# For vim:
+*.swp
+
+# For redcar:
+#.redcar
+
+# For rubinius:
+#*.rbc