implementing reading grade rule

.vale/fixtures/RedHat/ReadabilityGrade/testinvalid.adoc

@@ -0,0 +1 @@
+Before you can deploy virtual distributed unit (vDU) applications, you need to tune and configure the cluster host firmware and various other cluster configuration settings.

.vale/fixtures/RedHat/ReadabilityGrade/testvalid.adoc

@@ -0,0 +1 @@
+Tune and configure the cluster host firmware and various other cluster configuration settings.

.vale/styles/RedHat/ReadabilityGrade.yml

@@ -1,12 +1,7 @@
 ---
-extends: readability
-grade: 21
-level: suggestion
+extends: metric
+message: "Simplify your language. The calculated Flesch–Kincaid grade level (%s) is above the recommended value of 16."
 link: https://redhat-documentation.github.io/vale-at-red-hat/docs/main/reference-guide/readabilitygrade/
-message: "Grade level ('%s') too high."
-metrics:
-  - Automated Readability
-  - Coleman-Liau
-  - Flesch-Kincaid
-  - Gunning Fog
-  - SMOG
+formula: |
+  (0.39 * (words / sentences)) + (11.8 * (syllables / words)) - 15.59
+condition: "> 16"

modules/reference-guide/pages/readabilitygrade.adoc

@@ -3,9 +3,10 @@
 
 = Readability grade
 
-Grade level too high.
+Try to write sentences that have a calculated Flesch–Kincaid grade level above the recommended value of 15.
 
 .Additional resources
 
+* link:https://en.wikipedia.org/wiki/Flesch%E2%80%93Kincaid_readability_tests[Flesch–Kincaid readability tests]
 * link:https://vale.sh/docs/topics/styles#metric[Vale documentation - `metric` extension point]
 * link:{repository-url}blob/main/.vale/styles/RedHat/ReadabilityGrade.yml[`ReadabilityGrade.yml` source code]