Add doc about uploading addon source to existing addon version (#1601)

* doc about uploading addon source to existing addon version

* instructions text

Co-authored-by: rebloor <git@sherpa.co.nz>

* remove screenshot

* link to source code submission on "submitting an addon"

* "for more information"

Co-authored-by: rebloor <git@sherpa.co.nz>

---------

Co-authored-by: rebloor <git@sherpa.co.nz>

src/content/documentation/publish/source-code-submission.md

@@ -84,6 +84,10 @@ Code submission process happens after the add-on validation step.
 
 ![Upload sources screenshot](/assets/img/publish/upload-process-sources.png)
 
+#### Upload source for an existing version
+
+You can add source code to an existing add-on version. To do this, open [My Add-ons](https://addons.mozilla.org/en-US/developers/addons) and your add-on. In the left navigation, click **Manage Status & Versions**, then open the version you want to add source code to. At the bottom of the editor, next to **Source code**, click **Browse** and select the source code file. Make any other changes and then click **Save changes**.
+
 {% endcapture %}
 {% include modules/column-w-toc.liquid,
   id: "provide-your-extension-source-code"
@@ -162,5 +166,3 @@ Remember, if you miss any of the necessary content from your uploaded source cod
 %}
 
 <!-- END: Single Column Body Module -->
-
-

src/content/documentation/publish/submitting-an-add-on.md

@@ -75,7 +75,7 @@ Choose:
    Otherwise, you receive a message similar to this: <br/><br/> <img src="/assets/img/publish/errorsubmit.png" style="box-shadow:0px 0px 0.5em gray; height:50px; width:500px;" /> <br/>
    The message informs you of what failed. You'll not be able to continue. Address the issues and return to step 1.
 
-5. If you need to provide source code, click **Browse** and locate and upload your source code package.
+5. If you need to provide source code, click **Browse** and locate and upload your source code package. See [Source code submission](/documentation/publish/source-code-submission/) for more information.
 6. Select the add-on's compatible platform(s).
 7. Click **Continue**.
 8. The Describe Add-on page displays. <br/> ![Submit an add-on page](/assets/img/publish/Submit_add_on_describe_add_on.png) <br/>