Add gist_post_anonymous doc

Author: valeriangalliat

README.mkd

@@ -142,6 +142,10 @@ If you want your gist to be private by default:
 
     let g:gist_post_private = 1
 
+If you want your gist to be anonymous by default:
+
+    let g:gist_post_anonymous = 1
+
 If you want to manipulate multiple files in a gist:
 
     let g:gist_get_multiplefile = 1
@@ -240,3 +244,5 @@ list of ["Authorized applications" on GitHub's "Account Settings"
 page][uas].
 
 [uas]: https://github.com/settings/applications
+
+**Note:** the username is optional if you only send anonymous gists.

doc/gist-vim.txt

@@ -150,7 +150,10 @@ If you want to show your private gists with ":Gist -l": >
 If you want your gist to be private by default: >
 
     let g:gist_post_private = 1
+<
+If you want your gist to be anonymous by default: >
 
+    let g:gist_post_anonymous = 1
 <
 If you want to edit all files for gists containing more than one: >
 
@@ -270,6 +273,8 @@ Then, add following into your ~/.vimrc
 <
 This is not secure at all, so strongly discouraged.
 
+NOTE: the username is optional if you only send anonymous gists.
+
 ==============================================================================
 FAQ                                                           *gist-vim-faq*