I've just realised, this file is named .PKGINFO whereas all packages use PKGBUILD. PKGBUILD is the file that makepkg reads and builds the package according to it. .PKGINFO file is generated by makepkg command and it contains
pe, 2012-06-15 kello 23:14 +0100, Mateusz Loskot kirjoitti: list of files inside the package, it's size, name, version.... all that information. The .PKGINFO file is included inside the pkgname-pkgver-x86_64.pkg.tar.xz file and you should never modify it manually. .INSTALL is general name for the install file used in some packages. Basicly makepkg just copies the file specified in install= variable to the package along with the .PKGINFO file. Then about the .ChangeLog file. I am not 100% sure, but if I have not undestood everything incorrectly .ChangeLog file is the file specified in changelog= variable inside PKGBUILD and copied into the generated package by makepkg, just like .INSTALL file. This file should contain the changes made into the PKGBUILD and description on how they affect the application. ChangeLog without . in it is just the regular upstream Changelog file, that is then copyed into some dir in the package my make install or some cp/install command.