Add installation instructions for colcon-mixin

[colcon]

Add installation instructions for colcon-mixin

[cottsay]

Hello @KeisukeShima.

The colcon-mixin extension provides some great functionality, and while it is useful for many, I'm not sure it qualifies as a "common" extension. As the number of available colcon extensions grows, it increases the need for us to draw the line on what is included in the colcon-common-extensions metapackage.

That said, I think we need to improve awareness about colcon extensions which aren't part of colcon-common-extensions - a problem that I don't think is specific to colcon-mixin.

Maybe we should have a small snippet in the installation instructions redirecting users to a landing page in the colcon documentation, or maybe we could develop an extension for colcon itself to help users discover extensions that they don't currently have installed. I'm open to ideas here.

[KeisukeShima]

@cottsay Thank you for your comment. I think both of the ideas you suggested are great!

Discovering extensions that are not installed is a very good approach. However, as more colcon extensions are added, the hassle for users to install them will increase. How about creating a new metapackage to avoid that?

I think that colcon extensions can be classified into three types as follows:

1. Extension packages that currently exist in colcon-common-extensions
2. Extension packages provided by the colcon organization
3. Extension packages provided by other communities.

1 is an existing colcon-common-extension package.
My idea is to create a metapackage with 2 as an colcon-extra-extension. Then add it to the installation documentation. This will allow users to use the various colcon features out-of-the-box.
This is inspired by the ROS core and desktop package.

That said, I think we need to improve awareness about colcon extensions which aren't part of colcon-common-extensions - a problem that I don't think is specific to colcon-mixin.

Yes, I am aware that this is not limited to colcon-mixin. If there is a more appropriate place for discussion, I would like to move it there.