news-to-metainfo: add unstable release type for NEWS text format

[fedelibre]

In this blog post I read:

Unlike in NEWS files, [in NEWS.yml] releases can be defined as either stable or development depending on whether they are a stable or development release, by specifying a Type field. If no Type field is present, stable is implicitly assumed.

Would you consider adding this feature for the simple NEWS file as well? Otherwise all releases will be marked as stable even if they are not. I suggest something like this:

Version 2.1.3
~~~~~~~~~~
Released: 2021-09-23
Type: development

Description.....

[ximion]

I have nothing against adding something like this (PR welcome!), but did you know that you can mark a release as "in development" simply by using a release date with placeholders, such as Released: 2021-xx-xx already? Maybe that could be useful to you immediately.

[fedelibre]

I didn't know it. What about adding a "NEWS format" section in the man page? I think I can send a patch for this.

I guess that to implement Type: I should change something from here. I might give it a try if it's not too difficult for me.

[ximion]

I didn't know it. What about adding a "NEWS format" section in the man page? I think I can send a patch for this.

If that was added, I'd go for a more complete documentation, so, describe the expected formats in higher detail. That would probably be a bit much for appstreamcli's manual page. So having a dedicated manual page for this, or a page under the Miscellaneous chapter in the documentation may make more sense (the Misc section is still part of the spec though, so it wouldn't fir there 100%...)

I guess that to implement Type: I should change something from here. I might give it a try if it's not too difficult for me.

Should actually be a trivial change, if you know a bit of C and GLib (and yes, that's the right place to add this :-) )