It does not appear that there is any particular class or package devoted to Software Requirements Specifications and perhaps the reason is that this sort of document will follow an in-house format. It is not that difficult to develop one from scratch using one of the standard classes or any other class you are familiar with. I have used the KOMA-Script, report class for a short template to get you going. Here is the minimal code.
\documentclass{scrreprt}
\usepackage{listings}
\usepackage{underscore}
\usepackage[bookmarks=true]{hyperref}
\hypersetup{
bookmarks=false, % show bookmarks bar?
pdftitle={Software Requirement Specification}, % title
pdfauthor={Yiannis Lazarides}, % author
pdfsubject={TeX and LaTeX}, % subject of the document
pdfkeywords={TeX, LaTeX, graphics, images}, % list of keywords
colorlinks=true, % false: boxed links; true: colored links
linkcolor=blue, % color of internal links
citecolor=black, % color of links to bibliography
filecolor=black, % color of file links
urlcolor=purple, % color of external links
linktoc=page % only page is linked
}%
\def\myversion{1.0 }
\title{%
\flushright
\rule{16cm}{5pt}\vskip1cm
\Huge{SOFTWARE REQUIREMENTS\\ SPECIFICATION}\\
\vspace{2cm}
for\\
\vspace{2cm}
Materials Ordering System\\
\vspace{2cm}
\LARGE{Release 1.0\\}
\vspace{2cm}
\LARGE{Version \myversion approved\\}
\vspace{2cm}
Prepared by Yiannis Lazarides\\
\vfill
\rule{16cm}{5pt}
}
\date{}
\usepackage{hyperref}
\begin{document}
\maketitle
\tableofcontents
\chapter*{Revision History}
\chapter{Introduction}
\section{Purpose}
\section{Project Scope and Product Features}
\section{References}
\chapter{Overall Description}
\section{Product Perspective}
\section{User Classes and Characteristics}
% add other chapters and sections to suit
\end{document}
My own preference for such a document is to use XeLaTeX to process it, in order to have a wider selection of fonts.