Move vendoring to a separate page in the manual

manual/src/SUMMARY.md

@@ -7,6 +7,7 @@
   - [Mercurial](./mercurial.md)
 - [Parsing](./parsing.md)
   - [Upstream Parsers](./upstream_parsers.md)
+  - [Vendoring](./parser_vendoring.md)
   - [Adding A Parser](./adding_a_parser.md)
 - [Diffing](./diffing.md)
   - [Tricky Cases](./tricky_cases.md)

manual/src/adding_a_parser.md

@@ -10,10 +10,6 @@ parsers](https://tree-sitter.github.io/tree-sitter/#available-parsers).
 
 ## Add the source code
 
-Tree-sitter parsers are sometimes packaged on npm, sometimes packaged
-on crates.io, and have different release frequencies. Difftastic uses
-git subtrees (not git submodules) to track parsers.
-
 Once you've found a parser, add it as a git subtree to
 `vendor/`. We'll use
 [tree-sitter-json](https://github.com/tree-sitter/tree-sitter-json) as

manual/src/parser_vendoring.md

@@ -0,0 +1,23 @@
+# Vendoring
+
+## Git Subtrees
+
+Tree-sitter parsers are sometimes packaged on npm, sometimes packaged
+on crates.io, and have different release frequencies. Difftastic uses
+git subtrees (not git submodules) to track parsers.
+
+## Updating a parser
+
+To update a parser, pull commits from the upstream git repository. For
+example, the following command will update the Java parser:
+
+```
+$ git subtree pull --prefix=vendor/tree-sitter-java git@github.com:tree-sitter/tree-sitter-java.git master
+```
+
+To see when each parser was last updated, use the following shell
+command:
+
+```
+$ for d in $(git log | grep git-subtree-dir | tr -d ' ' | cut -d ":" -f2 | sort); do echo "$d"; git log --pretty="  %cs" -n 1 $d; done
+```