Add Readability package.

styles/Readability/AutomatedReadability.yml

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+extends: metric
+message: "Try to keep the Automated Readability Index (%s) below 8."
+link: https://en.wikipedia.org/wiki/Automated_readability_index
+
+formula: |
+  (4.71 * (characters / words)) + (0.5 * (words / sentences)) - 21.43
+
+condition: "> 8"

styles/Readability/ColemanLiau.yml

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+extends: metric
+message: "Try to keep the Coleman–Liau Index grade (%s) below 9."
+link: https://en.wikipedia.org/wiki/Coleman%E2%80%93Liau_index
+
+formula: |
+  (0.0588 * (characters / words) * 100) - (0.296 * (sentences / words) * 100) - 15.8
+
+condition: "> 9"

styles/Readability/FleschKincaid.yml

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+extends: metric
+message: "Try to keep the Flesch–Kincaid grade level (%s) below 8."
+link: https://en.wikipedia.org/wiki/Flesch%E2%80%93Kincaid_readability_tests
+
+formula: |
+  (0.39 * (words / sentences)) + (11.8 * (syllables / words)) - 15.59
+
+condition: "> 8"

styles/Readability/FleschReadingEase.yml

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+extends: metric
+message: "Try to keep the Flesch reading ease score (%s) above 70."
+link: https://en.wikipedia.org/wiki/Flesch%E2%80%93Kincaid_readability_tests
+
+formula: |
+  206.835 - (1.015 * (words / sentences)) - (84.6 * (syllables / words))
+
+condition: "< 70"

styles/Readability/GunningFog.yml

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+extends: metric
+message: "Try to keep the Gunning-Fog index (%s) below 10."
+link: https://en.wikipedia.org/wiki/Gunning_fog_index
+
+formula: |
+  0.4 * ((words / sentences) + 100 * (complex_words / words))
+
+condition: "> 10"

styles/Readability/LIX.yml

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+extends: metric
+message: "Try to keep the LIX score (%s) below 35."
+
+link: https://en.wikipedia.org/wiki/Lix_(readability_test)
+# Very Easy: 20 - 25
+#
+# Easy: 30 - 35
+#
+# Medium: 40 - 45
+#
+# Difficult: 50 - 55
+#
+# Very Difficult: 60+
+formula: |
+  (words / sentences) + ((long_words * 100) / words)
+
+condition: "> 35"

styles/Readability/SMOG.yml

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+extends: metric
+message: "Try to keep the SMOG grade (%s) below 10."
+link: https://en.wikipedia.org/wiki/SMOG
+
+formula: |
+  1.0430 * math.sqrt((polysyllabic_words * 30.0) / sentences) + 3.1291
+
+condition: "> 10"

styles/Readability/meta.json

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+{
+  "feed": "https://github.com/errata-ai/Readability/releases.atom",
+  "vale_version": ">=2.13.0"
+}