Yesterday 343 announced that Halo 5's Forge mode will come to Windows 10. Just earlier, Josh Holmes, studio head for Halo 5, confirmed that Forge for Windows 10 will include Custom Games: http://teambeyond.net/forum/topic/5443-halo-5-guardians-discussion/page-4803#entry767081
In regards to being up to date to it's Xbone counterpart: http://teambeyond.net/forum/topic/5443-halo-5-guardians-discussion/page-4804#entry767099
And if it will cost any money he simply said "Free" and that you can play Custom Games with Keyboard and Mouse.
We are giving players the ability to play full custom games to test their Forge creations on PC. Because Forge supports and builds upon existing Arena content, that includes the same maps and modes that are available in Arena on XB1. The intent of the experience is to allow Win10 users to Forge more effectively w/ keyboard and mouse and test their creations on PC before publishing to Xbox One. As such, there is no support for matchmaking or server browser. You will need to invite friends into a session through LIVE.
In regards to being up to date to it's Xbone counterpart: http://teambeyond.net/forum/topic/5443-halo-5-guardians-discussion/page-4804#entry767099
Yes. The nature of the experience (shared Forge content across XB1 & Win10) means we need to maintain both and keep them up to date & in sync with one another.
And if it will cost any money he simply said "Free" and that you can play Custom Games with Keyboard and Mouse.