update README (rules)

Author: andrew2net

README.adoc

@@ -125,6 +125,16 @@ map:
     "\u0412": "V"
 ----
 
+=== Rules
+
+The subsection `rules` is placed in the `map` section. All rules are applied in order they are placed before the subsection `characters` applying. Rules apply to an original text, not to a result of previous rules applying.
+
+Each rule has `pattern` and `result` elements.
+
+Pattern is a regex expression. It should be representing as a string without `//` or `%r{}` parentheses. For example `\b\u0415`. In case a rule is depend on previous or next content, lookahead or lookbehind could be used. For example a rule with the pattern `(?<=[АаЕеЁёИиОоУуЫыЭэЮюЯяЙйЪъЬь])\u0415` find every Е after upper or lower case symbols a, e, ё, и, о, у, ы, э, ю, я, й, ъ, ь.
+
+Result is a replacement a for pattrn's match. It can contain a string, an Unicode characters specified by a hexadecimal number, a captured group reference. String with hexadecimal number or captured group reference should be double quoted. For example `"Y\u00eb"` or `"\\1\u00b7\\2"`. Captured group are reffered by double backslash and group's number.
+
 === Testing transliteration systems
 
 To test all transliteration systems in `maps` direcrory run a command: