permissions - Missing support for Firebase App Indexing (android lint)

Question

I receive this lint warning when analysing my code (Analyse > Inspect Codes) on Android studios.

App is not indexable by Google Search; consider adding at least one Activity with an ACTION-VIEW intent-filler. See issue explanation for more details.

What is this warning, and how do I make my app indexable by Google Search? it sounds important for SEO, but I can't find any details on Google.

I also like to know how to access the "Issue Explanation" from android studio.

Edit:

"App is not indexable by Google Search" was the old warning. The new warning is "Missing support for Firebase App Indexing"

Answer 1

I found out how to access the "Issue Explanation". I need to hover over an inspection error to display the full issue explanation inline (and pressing Ctrl-F1)

so the keyword I am missing is "deep links"!

The following is the android developer page to do deep links "To enable Google to crawl your app content and allow users to enter your app from search results"

http://developer.android.com/training/app-indexing/deep-linking.html

the following is the code snippet on how to do a deep link. I got no idea how Google can crawl my app just by adding it though...

<activity
    android:name="com.example.android.GizmosActivity"
    android:label="@string/title_gizmos" >
    <intent-filter android:label="@string/filter_title_viewgizmos">
        <action android:name="android.intent.action.VIEW" />
        <category android:name="android.intent.category.DEFAULT" />
        <category android:name="android.intent.category.BROWSABLE" />
        <!-- Accepts URIs that begin with "http://www.example.com/gizmos” -->
        <data android:scheme="http"
              android:host="www.example.com"
              android:pathPrefix="/gizmos" />
        <!-- note that the leading "/" is required for pathPrefix-->
        <!-- Accepts URIs that begin with "example://gizmos”
        <data android:scheme="example"
              android:host="gizmos" />
        -->
    </intent-filter>
</activity>

there is also a note which says

Note: Intent filters may only contain a single data element for a URI pattern. 
Create separate intent filters to capture additional URI patterns.