runtime(doc): Add a Development policy

closes: #18197

Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/develop.txt

@@ -1,4 +1,4 @@
-*develop.txt*   For Vim version 9.1.  Last change: 2025 Sep 01
+*develop.txt*   For Vim version 9.1.  Last change: 2025 Sep 04
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -13,6 +13,7 @@ Vim.
 2. Design decisions	|design-decisions|
 3. Assumptions		|design-assumptions|
 4. Coding style		|coding-style|
+5. Policy		|design-policy|
 
 See the file README.txt in the "src" directory for an overview of the source
 code.
@@ -728,4 +729,35 @@ OK:	    do
 	    while (cond);
 
 
+==============================================================================
+5. Policy	*design-policy* *new-features* *deprecated-features*
+
+The time between either a new minor (e.g. 9.2.0) or major (e.g. 10.0) version
+is released is called a development cycle.  Within the development cycle each
+single change to the C core will receive a new increased human-readable patch
+number in order to reference each specific patch release.  A typical
+development release cycle may last several years and accumulate about 1500 -
+2500 patch numbers.
+
+Before a release is made, a stability period will be announced. During this
+time, only clear bug fixes, security fixes, documentation changes, translation
+updates and runtime file updates will be accepted (provided they do not
+introduce backwards-incompatible changes), concentrating on polishing up the
+upcoming release.
+
+New features are accepted only within a development cycle, but not within the
+stability period.  During the cycle, new features may be developed and are
+allowed to change, but they must be settled before the cycle closes.
+
+Once a minor release has been made, features included in that release must not
+receive any backwards-incompatible changes.  Later patches are expected to
+preserve compatibility for the C core of Vim.  Runtime files are handled a bit
+more flexibly to give runtime files maintainers a chance to change old
+behaviour.
+
+Within a development cycle, features may be marked as deprecated. Deprecated
+features can be disabled at compile time through an appropriate switch. After
+a new release, deprecated features may be removed completely in a following
+cycle.
+
  vim:tw=78:ts=8:noet:ft=help:norl:

runtime/doc/tags

@@ -6969,6 +6969,7 @@ delete-menus	gui.txt	/*delete-menus*
 deletebufline()	builtin.txt	/*deletebufline()*
 deleting	change.txt	/*deleting*
 demoserver.py	channel.txt	/*demoserver.py*
+deprecated-features	develop.txt	/*deprecated-features*
 design-assumptions	develop.txt	/*design-assumptions*
 design-compatible	develop.txt	/*design-compatible*
 design-decisions	develop.txt	/*design-decisions*
@@ -6979,6 +6980,7 @@ design-improved	develop.txt	/*design-improved*
 design-maintain	develop.txt	/*design-maintain*
 design-multi-platform	develop.txt	/*design-multi-platform*
 design-not	develop.txt	/*design-not*
+design-policy	develop.txt	/*design-policy*
 design-speed-size	develop.txt	/*design-speed-size*
 desktop.vim	syntax.txt	/*desktop.vim*
 develop-spell	develop.txt	/*develop-spell*
@@ -9367,6 +9369,7 @@ new-diff-mode	version6.txt	/*new-diff-mode*
 new-encryption	version5.txt	/*new-encryption*
 new-evim	version6.txt	/*new-evim*
 new-ex-commands-5.2	version5.txt	/*new-ex-commands-5.2*
+new-features	develop.txt	/*new-features*
 new-file-browser	version6.txt	/*new-file-browser*
 new-file-writing	version6.txt	/*new-file-writing*
 new-filetype	filetype.txt	/*new-filetype*

runtime/doc/vi_diff.txt

@@ -1,4 +1,4 @@
-*vi_diff.txt*   For Vim version 9.1.  Last change: 2025 Sep 02
+*vi_diff.txt*   For Vim version 9.1.  Last change: 2025 Sep 04
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar