Make README.md file a requirement

Before the README file was not a requirement. But I think it is important that each package contains one. This also makes it possible to validate it is there. In the case of the system package, the README was there but with a typo and we didn't detect it.

CHANGELOG.md

@@ -17,6 +17,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 
 * Allow to set cache times through config. [#271](https://github.com/elastic/integrations-registry/pull/271)
+* Make README.md file a required file for a package. [#287](https://github.com/elastic/integrations-registry/pull/287)
 
 ### Deprecated
 

dev/packages/example/base-1.0.0/docs/README.md

@@ -0,0 +1,3 @@
+# Base package
+
+This is installed in the background by EPM to setup the Elastic Stack.

dev/packages/example/yamlpipeline-1.0.0/docs/README.md

@@ -0,0 +1,3 @@
+# YAML Pipeline
+
+Example package with a yaml pipeline.

util/package.go

@@ -134,8 +134,8 @@ func NewPackage(basePath, packageName string) (*Package, error) {
 	readmePath := basePath + "/" + packageName + "/docs/README.md"
 	// Check if readme
 	readme, err := os.Stat(readmePath)
-	if err != nil && !os.IsNotExist(err) {
-		return nil, err
+	if err != nil {
+		return nil, fmt.Errorf("no readme file found, README.md is required: %s", err)
 	}
 
 	if readme != nil {