Add documentation for org-crypt.el

Patch by Ian Barton.

doc/org.texi

@@ -12911,6 +12911,7 @@ emacs -Q --batch -l $ORGINSTALL \
 * Clean view::                  Getting rid of leading stars in the outline
 * TTY keys::                    Using Org on a tty
 * Interaction::                 Other Emacs packages
+* org-crypt.el::                Encrypting Org files
 @end menu
 
 
@@ -13588,7 +13589,7 @@ tty you would rather use @kbd{C-c .} to re-insert the timestamp.
 @end multitable
 
 
-@node Interaction,  , TTY keys, Miscellaneous
+@node Interaction, org-crypt.el, TTY keys, Miscellaneous
 @section Interaction with other packages
 @cindex packages, interaction with other
 Org lives in the world of GNU Emacs and interacts in various ways
@@ -13797,6 +13798,33 @@ another key for this command, or override the key in
 
 @end table
 
++@node org-crypt.el,  , Interaction, Miscellaneous
++@section org-crypt.el
++@cindex @file{org-crypt.el}
++@cindex @code{org-decrypt-entry}
+
+Org-crypt will encrypt the text of an entry, but not the headline, or
+properties. Org-crypt uses the Emacs EasyPG library to encrypt and decrypt
+files.
+
+Any text below a headline that has a @samp{:crypt:} tag will be
+automatically be encrypted when the file is saved. If you want to use a
+different tag just customize the @code{org-crypt-tag-matcher} setting.
+
+To use org-crypt it is suggested that you have the following in your
+@file{.emacs}:
+
+@example
+(require 'org-crypt)
+(org-crypt-use-before-save-magic)
+(setq org-tags-exclude-from-inheritance (quote ("crypt")))
+;; GPG key to use for encryption
+;; Either the Key ID or set to nil to use symmetric encryption.
+(setq org-crypt-key nil)
+@end example
+
+Excluding the crypt tag from inheritance prevents already encrypted text
+being encrypted again.
 
 @node Hacking, MobileOrg, Miscellaneous, Top
 @appendix Hacking