added doc and changelog entry for preserve-tombs plugin

docs/changelog.txt

@@ -52,6 +52,7 @@ Template for new versions:
 # Future
 
 ## New Tools
+- `preserve-tombs`: tracks tomb assignments to living units and ensures that the tomb stays assigned to them when they die.
 
 ## New Features
 

docs/plugins/preserve-tombs.rst

@@ -0,0 +1,22 @@
+preserve-tombs
+==============
+
+.. dfhack-tool::
+    :summary: Fix tombs being unassigned to units on death
+    :tags: fort bugfix
+
+If you find that the tombs you assign to units get unassigned from them when
+they die (e.g. your nobles), this tool can help fix that.
+
+Usage
+-----
+
+::
+
+    enable preserve-tombs
+    preserve-tombs status
+
+This tool runs in the background. You can check the status of the plugin
+by running ``preserve-tombs status`` which will show whether the plugin
+is enabled and if so, display a list of all tracked tomb assignments
+to living units.