Possessive case
 

Hi,

They have been discussing this particular case for decades, yet I can't remember what is the final agreement :)

There is a Peacemaker project, for instance.

I'd like to create the Peacemaker Developers Guide. Can I get away with this, or some apostrophe is indispensable?

WARNING: Controversial Topic (there's more than one "right" answer)

You'll certainly see 'Developers Guide' used, but prescriptively speaking, this is clearly an example of the genitive case; it's not a possessive because I don't believe the guide belongs to the individual or individuals in question. It may be theirs to keep, but we're talking about the title of your guide, not the guide itself.

The question is where the apostrophe should be placed.

The word 'developer' is not a proper noun, so that saves us from a weightier discussion. (We don't have to debate whether it's "James' car" or "James's car.")

The word 'developer' is a singular noun.
To form its plural we add "s."

To form the genitive, we add "'s" to a singular noun.
For regular plural nouns (like 'developers'), we add only an apostrophe.

So, depending on whether the noun is singular or plural, we either have the genitive "developer's" or "developers'."

I would like to think that you've designed your guide for more than one developer (they don't all have to be in the same room at the same time).
Given that assumption, your title should be "Peacemaker Developers' Guide."