[x265] [PATCH] doc: Update build/readme.txt for detailed instructions with nasm

[Pradeep Ramachandran]

# HG changeset patch
# User Pradeep Ramachandran <pradeep at multicorewareinc.com>
# Date 1512016609 -19800
#      Thu Nov 30 10:06:49 2017 +0530
# Node ID b1dfa312234ed72c3541831a15f307feaf79484d
# Parent  9eabffb26dd62e4f48c5679594ae13690eb9d221
doc: Update build/readme.txt for detailed instructions with nasm

diff -r 9eabffb26dd6 -r b1dfa312234e build/README.txt
--- a/build/README.txt	Thu Nov 02 09:40:41 2017 +0530
+++ b/build/README.txt	Thu Nov 30 10:06:49 2017 +0530
@@ -9,7 +9,8 @@
 
 = Optional Prerequisites =
 
-1. Yasm 1.2.0 or later, to compile assembly primitives (performance)
+1. To compile assembly primitives (performance) 
+   a) If you are using release 2.6 or older, download and install Yasm 1.2.0 or later, 
 
    For Windows, download the latest yasm executable
    http://yasm.tortall.net/Download.html and copy the EXE into
@@ -33,6 +34,24 @@
    If cpu capabilities line says 'none!', then the encoder was built
    without yasm.
 
+   b) If you are building from the default branch after release 2.6, download and install nasm 2.13 or newer
+   
+   For windows and linux, you can download the nasm installer from http://www.nasm.us/pub/nasm/releasebuilds/?C=M;O=D.
+   Make sure that it is in your PATH environment variable (%PATH% in windows, and $PATH in linux) so that cmake
+   can find it.
+
+   Once NASM is properly installed, run cmake to regenerate projects. If you
+   do not see the below line in the cmake output, NASM is not in the PATH.
+
+   -- Found Nasm 2.13 to build assembly primitives
+
+   Now build the encoder and run x265 -V:
+
+   x265 [info]: using cpu capabilities: MMX, SSE2, ...
+
+   If cpu capabilities line says 'none!', then the encoder was built
+   without nasm and will be considerably slower for performance.
+
 2. VisualLeakDetector (Windows Only)
 
    Download from https://vld.codeplex.com/releases and install. May need