Add comment about live at head philosophy

[jmigual]

Since this repository considers master as the latest release (this is called Abseil Live at Head philosophy) I added a comment to the README explaining it.

[gregsdennis]

I'm not sure that "live at head" is really something that comes from that specific project.

Do we really need to call out that the main branch is the main branch? In my experience, using it this way the default strategy for most projects. In fact, this is the default for GitHub.

[jmigual]

In many projects the main branch is "unstable" until a proper release has been made so projects depending on this one only look at the releases part. From issue #672 I understood that in this project main is always in a stable state. That's why I'm making this PR. Also, probably unrelated, but due to some misconfiguration the latest release was not visible to everyone.

One project that was waiting unnecessarily for a stable release without looking at "main" was JSONSchema.jl as discussed here

[jmigual]

Do we really need to call out that the main branch is the main branch?

The purpose of adding this to the README was not to call out that the main branch is the main branch but rather than in this project the main branch is always stable which is not always the case.

[Julian]

Greg is, I believe, referring to you including a reference to Abseil, which I too agree isn't needed. I've also never heard of Abseil and also am quite confident it didn't invent this kind of thing. (I also wouldn't include the phrase "live at head", which isn't really a common phrase, or at least I've never heard it either).

We should just say, matter of fact-ly, that the way to use this repository is generally by including it as a git submodule or git subtree.

[jmigual]

Alright I changed it to a better simpler description.

I initially heard about Abseil Live at Head philosophy in Google Test repository and assumed that many people would've heard it before as well (abseil's c++ guidelines are quite known in the c++ community) but I agree that this may not be the case to everyone.

I hope this suits better with the intentions of the repository :)

[Julian]

Great, thanks @jmigual this looks perfect I think now -- really appreciate taking the time to add it! Let us know if you have other suggestions.