lint: Check for README presence

After exercism/meta#15, READMEs are to be
generated and placed into each track's exercise implementation
directory.

My (unsubstantiated) assumption is that they will be required to be
present after Nextercism, as we don't want to keep generating READMEs on
the fly.

If this assumption is correct, it seems necessary to check that READMEs
are present on all exercises.

With the attached fixture change and attached test changes:

* The added TestMissingReadme would fail if the attached code change
  were incorrect.
* The added TestLintTrack case on missing-readmes would fail without
  the attached code change.

Closes exercism/discussions#200
Closes exercism/configlet#86

cmd/lint.go

@@ -89,6 +89,10 @@ func lintTrack(path string) bool {
 			check: missingMetadata,
 			msg:   "An implementation for '%v' was found, but config.json does not reference this exercise.",
 		},
+		{
+			check: missingReadme,
+			msg:   "The implementation for '%v' is missing a README.",
+		},
 		{
 			check: missingSolution,
 			msg:   "The implementation for '%v' is missing an example solution.",
@@ -201,6 +205,25 @@ func missingSolution(t track.Track) []string {
 	return slugs
 }
 
+func missingReadme(t track.Track) []string {
+	readmes := map[string]bool{}
+	for _, exercise := range t.Exercises {
+		readmes[exercise.Slug] = exercise.HasReadme()
+	}
+	// Don't complain about missing readmes in foregone exercises.
+	for _, slug := range t.Config.ForegoneSlugs {
+		readmes[slug] = true
+	}
+
+	slugs := []string{}
+	for slug, ok := range readmes {
+		if !ok {
+			slugs = append(slugs, slug)
+		}
+	}
+	return slugs
+}
+
 func missingTestSuite(t track.Track) []string {
 	tests := map[string]bool{}
 	for _, exercise := range t.Exercises {

cmd/lint_test.go

@@ -45,6 +45,11 @@ func TestLintTrack(t *testing.T) {
 			path:     "../fixtures/broken-maintainers",
 			expected: true,
 		},
+		{
+			desc:     "should fail when given a track missing READMEs.",
+			path:     "../fixtures/missing-readmes",
+			expected: true,
+		},
 		{
 			desc:     "should not fail when given a track with all of its bits in place.",
 			path:     "../fixtures/lint/valid-track",
@@ -110,6 +115,27 @@ func TestMissingMetadata(t *testing.T) {
 	assert.Equal(t, "cherry", slugs[1])
 }
 
+func TestMissingReadme(t *testing.T) {
+	track := track.Track{
+		Exercises: []track.Exercise{
+			{Slug: "apple"},
+			{Slug: "banana", ReadmePath: "README.md"},
+			{Slug: "cherry"},
+		},
+	}
+
+	slugs := missingReadme(track)
+
+	if len(slugs) != 2 {
+		t.Fatalf("Expected missing READMEs in 2 exercises, missing in %d", len(slugs))
+	}
+
+	sort.Strings(slugs)
+
+	assert.Equal(t, "apple", slugs[0])
+	assert.Equal(t, "cherry", slugs[1])
+}
+
 func TestMissingSolution(t *testing.T) {
 	track := track.Track{
 		Exercises: []track.Exercise{

fixtures/missing-readme/config.json

@@ -0,0 +1,13 @@
+{
+  "slug": "missing-readme",
+  "language": "Missing Readme",
+  "repository": "https://github.com/exercism/missing-readme",
+  "active": true,
+  "exercises": [
+    {
+      "slug": "missing-readme",
+      "topics": [],
+      "difficulty": 1
+    }
+  ]
+}