add markdown_proofer

Gemfile

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+source 'https://rubygems.org'
+
+gem 'markdown_proofer'
+gem 'rake'

Gemfile.lock

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+GEM
+  remote: https://rubygems.org/
+  specs:
+    activesupport (4.1.1)
+      i18n (~> 0.6, >= 0.6.9)
+      json (~> 1.7, >= 1.7.7)
+      minitest (~> 5.1)
+      thread_safe (~> 0.1)
+      tzinfo (~> 1.1)
+    colored (1.2)
+    ethon (0.7.0)
+      ffi (>= 1.3.0)
+    ffi (1.9.3)
+    github-markdown (0.6.5)
+    html-pipeline (1.8.0)
+      activesupport (>= 2)
+      nokogiri (~> 1.4)
+    html-proofer (0.6.8)
+      colored (~> 1.2)
+      mercenary (~> 0.3.2)
+      nokogiri (~> 1.6.0)
+      typhoeus (~> 0.6.7)
+      yell (~> 2.0)
+    i18n (0.6.9)
+    json (1.8.1)
+    markdown_proofer (0.1.0)
+      github-markdown (~> 0.6.5)
+      html-pipeline (~> 1.8)
+      html-proofer (~> 0.6.7)
+    mercenary (0.3.3)
+    mini_portile (0.6.0)
+    minitest (5.3.4)
+    nokogiri (1.6.2.1)
+      mini_portile (= 0.6.0)
+    rake (10.3.2)
+    thread_safe (0.3.3)
+    typhoeus (0.6.8)
+      ethon (>= 0.7.0)
+    tzinfo (1.1.0)
+      thread_safe (~> 0.1)
+    yell (2.0.4)
+
+PLATFORMS
+  ruby
+
+DEPENDENCIES
+  markdown_proofer
+  rake

Rakefile

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+require 'rubygems'
+require 'bundler'
+Bundler.require(:default)
+
+desc "Run Mardown validation for the repository"
+task :validate_markdown do
+  MarkdownProofer::RakeTask.run
+end
+
+task default: :validate_markdown