README.md: write about kind letters and names

Signed-off-by: Masatake YAMATO <yamato@redhat.com>

README.md

@@ -1,4 +1,3 @@
-
 # Universal Ctags
 
 [![Build Status](https://travis-ci.org/universal-ctags/ctags.svg?branch=master)](https://travis-ci.org/universal-ctags/ctags)
@@ -57,18 +56,37 @@ In addition rst2man command is needed.
 
 rst2man is part of the python-docutils package on Ubuntu.
 
-## Difference ##
+## Differences ##
 
 You may be interested in how universal-ctags is different from
 exuberant-ctags. The critical and attractive changes are explained
 in docs/\*.rst. The preformatted version is available on line,
 http://docs.ctags.io/.
 
-The most significant one is that Universal-ctags doesn't load
+The most significant incompatible changes:
+
+* Universal-ctags doesn't load
 `~/.ctags` and `./.ctags` at starting up time. Instead, it loads
 `~/.ctags.d/*.ctags` and `./.ctags.d/*.ctags`. See the above web
 site and man pages
 (man/ctags.1.rst.in and man/ctags-incompatibilities.7.in in the
 source tree).
 
+* Universal-ctags is more strict about characters that can be
+  used in kind letters and kind names than Exuberant-ctags.
+
+  - The letter must be an alphabetical character (`[a-zA-EG-Z]`).
+    `F` is reserved for `file` kind.
+
+  - The first character of the name must be alphabetic, and
+    the rest characters must be alphanumeric (`[a-zA-Z][a-zA-Z0-9]*`).
+
+  See the web site and man pages. The detailed background is explained
+  in [#1737](https://github.com/universal-ctags/ctags/pull/1737).
+
+  If you want to reuse your .ctags written for Exuberant-ctags,
+  you must review kind letters and names defined with `--regex-<LANG>=...`
+  options. When updating the definitions, using `--kind-<LANG>` option
+  is appreciated.
+
 Pull-requests are welcome!