README: Add more info about zone parser

README

 Dependencies
 ============
+
 Knot DNS has several dependencies:
 * libtool
 * autoconf > 2.65
@@ -9,16 +10,23 @@ Knot DNS has several dependencies:
 * liburcu >= 0.5.4
 
 Optional packages:
-* libcap-ng >= 0.6.4 for POSIX 1003.1e capabilites(7)
-* ragel
+* libcap-ng >= 0.6.4 (for POSIX 1003.1e capabilites(7))
+* ragel (for fast zone parsing)
 
 Dependencies for building documentation:
 * texinfo
 
 Knot DNS requires compiler to support atomic intrinsics.
-GCC version at least 4.1 supports legacy atomic builtins, however 4.7 or newer is preferred.
+GCC version at least 4.1 supports legacy atomic builtins, however 4.7
+or newer is preferred.
 Clang supports atomics since version 2.9.
 
+By default Knot DNS is distributed with a slower zone file parser because of
+smaller source file and quick compile time. In most cases it is sufficient.
+If you plan to process large zone files, it is recomended to build Knot DNS
+using ./configure --enable-fastparser. In this case Ragel compiler is required
+and it is normal if the C compiler takes much more time (minutes).
+
 Installation
 ============
 
@@ -75,7 +83,6 @@ $ wget http://bd.archive.ubuntu.com/ubuntu/pool/universe/libu/liburcu/liburcu-de
 $ sudo dpkg -i liburcu0_0.5.4-1_i386.deb
 $ sudo dpkg -i liburcu-dev_0.5.4-1_i386.deb
 
-
 Installation on Fedora
 ======================
 
@@ -132,6 +139,7 @@ $ sudo make install
 
 Installation on OpenBSD/NetBSD
 ==============================
+
 Also works for OS X, if you don't want to install gcc from ports.
 Prerequisites:
 - liburcu needs patch for absent compiler TLS capability