Update repositories.asciidoc

Added a warning explaining how `add-apt-repository` adds a `deb-src` entry, which can result in errors.

Closes #10223

docs/reference/setup/repositories.asciidoc

@@ -30,6 +30,18 @@ Add the repository definition to your `/etc/apt/sources.list` file:
 echo "deb http://packages.elasticsearch.org/elasticsearch/{branch}/debian stable main" | sudo tee -a /etc/apt/sources.list
 --------------------------------------------------
 
+[WARNING]
+==================================================
+Use the `echo` method described above to add the Elasticsearch repository.  Do not use `add-apt-repository` 
+as it will add a `deb-src` entry as well, but we do not provide a source package.  
+If you have added the `deb-src` entry, you will see an error like
+the following:
+
+    Unable to find expected entry 'main/source/Sources' in Release file (Wrong sources.list entry or malformed file)
+    
+Just delete the `deb-src` entry from the `/etc/apt/sources.list` file and the installation should work as expected.
+==================================================
+
 Run apt-get update and the repository is ready for use. You can install it with :
 
 [source,sh]