Add README.md

Author: k-takata

README.md

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+Vim-history
+-----------
+
+This is a repository for archiving very very old source code of Vim.
+This includes the history from v1.14 to v5.5.
+
+This is created from the tarballs in the Vim's FTP site:
+
+* <http://ftp.vim.org/pub/vim/old/>  
+  (v1.14 - v2.8, v4.0 - v4.5)
+* <http://ftp.vim.org/pub/vim/unix/>  
+  (v3.0, v4.6, v5.0 - v5.5)
+
+So this is not the TRUE history. (E.g. commit dates are faked. They are based
+on the dates from `src/version.c` or `src/version.h`.)
+
+
+Other repositories
+------------------
+
+* vim-ancient  
+  <https://bitbucket.org/vim-mirror/vim-ancient>  
+  This includes the history from v5.5 to v6.4.  
+  This was converted from the old svn repository.
+
+* The official repository  
+  <https://github.com/vim/vim>  
+  This includes the history from v7.0 to the latest release.