Hammond and Scull (the authors of the LotR companion), have written a Silmarillion companion inside one of their other books.
Wayne Hammond and Christina Scull, the Tolkien Scholars who wrote The Lord of the Rings A Reader's Companion, have also included a lengthy guide to The Silmarillion as part of their JRR Tolkien Companion and Guide. There's a long intro section about the work as a whole, and then there are individual entrees for each chapter. All in all I counted over 150 pages devoted to The Silmarillion.
Christina has said that she considers this to be the equivalent of a reader's guide.
We still want The Silmarillion A Reader's Companion by Wayne Hammond and Christina Scull. Please do it.
Wayne: That would be interesting because you have a lot of connections with The History of Middle-earth. We did suggest in fact an Art of the Silmarillion, but of course there's a lot less of original art for that.
Christina: But to a certain extent there is a guide to The Silmarillion within the Companion and Guide. It was very very difficult sorting out how to deal with The Silmarillion. Because you had to deal with the summary of The History of Middle-earth. It took me a long while, thinking about how I would tackle it. I did that. I worked on that while Wayne was still doing another bibliography for another author. And it eventually came around that I would have to it chapter by chapter of the Silmarillion, with individual entrees for every element of it, and then do the chapters. "No I can't start from the start, because there won't be a start to start with for certain aspects of it". So give the summary of the Silmarillion chapter and then go back and work all the way up [Wayne: Trace the history of it] and it took a lot of work on that within the things. It's almost a guide to it. But of course, that was done before all the elements were [Wayne: Twenty years ago] that we had to bring it up do date. That was the things we had to bring ourselves up to date on.
Tolkien Talk 559 - Interview with Wayne G. Hammond & Christina Scull [1:33:30-1:35:17]
Also as noted by the other answers, volumes 1-5 & 10-11 of The History Of Middle-earth collect a lot of the drafts that went into the Silmarillion, and Kane's Arda Reconstructed compares those drafts to the published book. But none of that would really classify as a "companion" or "guide" to the book.