chore(gatsby-plugin-feed): Update README with clearer instructions

[geocine]

Description

link is not properly being replaced because the underlying option used by node-rss is site_url so I just created a mapping. custom_namespaces is another option used in node-rss to specify custom namespaces.

Documentation

link is already in documentation. added custom_namespaces to the documentation.

Tests

I clone a copy of this plugin using this process to test https://www.gatsbyjs.com/docs/creating-a-local-plugin/

Related Issues

[LekoArts]

I removed your addition to the plugin options because it's already covered by this unknown here:

gatsby/packages/gatsby-plugin-feed/src/plugin-options.js

Line 13 in 0e9feeb

.unknown(true)

I updated the README though, let me know if this is clearer to you.

[geocine]

I removed your addition to the plugin options because it's already covered by this unknown here:

gatsby/packages/gatsby-plugin-feed/src/plugin-options.js

Line 13 in 0e9feeb

.unknown(true)

I updated the README though, let me know if this is clearer to you.

Thank you, that make its clearer. I added custom_namespaces because I didn't want to see warnings.

[LekoArts]

I've tried it locally with an example site of mine, when I use e.g. language inside feeds I don't get a warning.