lint: add annotation for key features in config.json

If a track's `config.json` file does not have valid key features,
the error message will now also show an annotation which explains
to the user what exactly is wrong and how to fix it.

Author: ErikSchierboom

src/lint/track_config.nim

@@ -175,17 +175,52 @@ const keyFeatureIcons = [
 ].toHashSet()
 
 proc isValidKeyFeature(data: JsonNode; context: string; path: Path): bool =
+  const iconErrorAnnotation = """
+    A key feature's `icon` is shown for the feature when presented on the website.
+    The icon must be chosen from our list of supported icons:
+    https://exercism.org/docs/building/tracks/icons#h-key-feature-icons
+    You can choose any icon that you think fits, regardless of its name.
+
+    For more information on key features see:
+    https://exercism.org/docs/building/tracks/config-json#h-key-features""".unindent()
+
+  const titleErrorAnnotation = """
+    A key feature's `title` is a concise header for the key feature.
+    As little technical jargon as possible should be used.
+    Its length must be <= 25 and Markdown is not supported.
+
+    For more information on key features see:
+    https://exercism.org/docs/building/tracks/config-json#h-key-features""".unindent()
+
+  const contentErrorAnnotation = """
+    A key feature's `content` is a description of the key feature.
+    Its length must be <= 100 and Markdown is not supported.
+
+    For more information on key features see:
+    https://exercism.org/docs/building/tracks/config-json#h-key-features""".unindent()
+ 
   if isObject(data, context, path):
     let checks = [
-      hasString(data, "icon", path, context, allowed = keyFeatureIcons),
-      hasString(data, "title", path, context, maxLen = 25),
-      hasString(data, "content", path, context, maxLen = 100),
+      hasString(data, "icon", path, context, allowed = keyFeatureIcons,
+                errorAnnotation = iconErrorAnnotation),
+      hasString(data, "title", path, context, maxLen = 25,
+                errorAnnotation = titleErrorAnnotation),
+      hasString(data, "content", path, context, maxLen = 100,
+                errorAnnotation = contentErrorAnnotation),
     ]
     result = allTrue(checks)
 
 proc hasValidKeyFeatures(data: JsonNode; path: Path): bool =
+  const errorAnnotation = """
+    The key features succinctly describe the most important features
+    of the language to promote the language to potential students.
+    Exactly 6 key features must be specified.
+
+    For more information on key features see:
+    https://exercism.org/docs/building/tracks/config-json#h-key-features""".unindent()
   result = hasArrayOf(data, "key_features", path, isValidKeyFeature,
-                      isRequired = false, allowedLength = 6..6)
+                      isRequired = false, allowedLength = 6..6,
+                      errorAnnotation = errorAnnotation)
 
 const tags = [
   "paradigm/declarative",