add some documentation for the svn plugin

Plugin:svn.md

@@ -0,0 +1,45 @@
+
+###Agnoster theme git-like prompt
+
+Enable the svn plugin and add the followind lines to your ```~/.zshrc```
+
+```shell
+prompt_svn() {
+    local rev branch
+    if in_svn; then
+        rev=$(svn_get_rev_nr)
+        branch=$(svn_get_branch_name)
+        if [ `svn_dirty_choose_pwd 1 0` -eq 1 ]; then
+            prompt_segment yellow black
+            echo -n "$rev@$branch"
+            echo -n "±"
+        else
+            prompt_segment green black
+            echo -n "$rev@$branch"
+        fi
+    fi
+}
+```
+
+```shell
+build_prompt() {
+    RETVAL=$?
+    prompt_status
+    prompt_context
+    prompt_dir
+    prompt_git
+    prompt_svn
+    prompt_end
+}
+```
+
+## Functions
+
+
+| Command                | Description                             |
+|:-----------------------|:----------------------------------------|
+|svn_prompt_info         | prompt for some themes                  |
+|in_svn                  | within svn directory                    |
+|svn_get_repo_name       |                                         |
+|svn_get_branch_name     | branch name (bug: returns the first path element if branches not used) |
+|svn_dirty               | changes in this subversion repo         |