docs: Add a Jujutsu from first principles doc.

This document is WIP. 

This came up recently in Discord as a explanation was requested without referencing Git, which
all our other documents do.

docs/jj_first_principles.md

@@ -0,0 +1,60 @@
+# Jujutsu from first principles (without Git)
+
+## Preface
+
+Why does Jujutsu exist and which problems does it solve? This document tries to
+answer both of these questions while expanding on the design in a user-friendly
+way.
+
+At its core Jujutsu is [Version Control System][vcs] which scales to huge 
+repositories at [Google scale][billion-lines]. Many design choices are 
+influenced by the concurrent commits happening in Googles Monorepo, as there 
+are always multiple people working on the same file(s) at the same time.
+
+## Core Tenets
+
+Jujutsu's core tenets are:
+
+ * User-friendliness: Making the  working copy a commit is simpler. This is 
+ how the project started.
+ * The "repository", so the commit graph is the source of truth. The working 
+ copy is just one way of editing commits.
+ * All operations must be able to scale to Google-scale repos (lots of commits
+ , lots of files): Laziness is important, must avoid accessing data 
+ unnecessarily.
+ * Having as few states as possible.
+ * Make it incredibily hard to lose work in your repository.
+ * Allow concurrent edits on any commit, pending or finished.
+ * Make a "stacked diffs" workflow as easy as possible.
+ * Git-interop: Git is everywhere. We need to have good interop to be adapted.
+ * Pluggable storage: Must be easy to integrate with different commit storage,
+ virtual file systems and more.
+
+## Base design
+
+The initial base design is to be a conceptually simpler Mercurial, as 
+automatically snapshotting the working copy simplifies the UX of the 
+command-line interface by a huge amount and avoids many bad states.
+
+By also choosing to operate by default on the history of the repository (
+just called the "the Graph" from now on) instead of files, all history 
+modifying commands can be done at any point. This is a major improvement on 
+other version control systems as they need to re-apply a single patch on each 
+new ancestor before finishing the Graph rewrite. Since the Graph can be changed
+at any point, the working copy cannot contain any state depending on it, thus 
+we have the working-copy commit, which just is another commit from the Graph's
+point of view. 
+
+
+### Change-IDs and Changes
+
+Since Jujutsu is oriented around a "stacked diffs" kind of workflow, which 
+primarily work on individually versiond patch sets, some kind of container is 
+needed, this is what a Change is. They are provided with a unique id to address
+them easily. 
+
+TODO: expand on change-ids and "working on the graph/repo" instead of a commit
+
+
+[billion-lines]: https://www.youtube.com/watch?v=W7*TkUbdqE&t=327s
+[vcs]: https://en.wikipedia.org/wiki/Version_control 

mkdocs.yml

@@ -86,6 +86,8 @@ nav:
 
 - FAQ: 'FAQ.md'
 
+- 'Jujutsu from first principles': 'jj_first_principles.md'
+
 - "CLI Reference": 'cli-reference.md'
 
 - Testimonials: 'testimonials.md'