--- a/README
+++ b/README
@@ -54,3 +54,21 @@ Pull new revisions into an already-conve
  $ hg svn pull
 
 For more information, see 'hg help svn' while in a converted repo.
+
+Support for svn:externals
+-------------------------
+All svn:externals properties are serialized into a single .hgsvnexternals file, with the following syntax:
+
+--------------------
+[.]
+ external_reference_line1_1
+ external_reference_line1_2
+[dir2]
+ external_reference_line2_1
+ external_reference_line2_2
+--------------------
+
+A header line gives the directory the property applies on, '.' for the project root directory. The property content follows, *every line being prefixed by a single space*.
+
+The creation or modification of this file will trigger svn:externals updates on the related subversion repository.
+