New CHANGELOG.txt and (Linux) INSTALL files

Name and location of INSTALL file may be subject to more discussion

INSTALL

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+Compilation and install instructions for GNU/Linux
+
+Download wxWidgets. Audacity 2.1.3 requires wxWidgets 3.0.x, preferably 3.0.2, 
+which can be obtained from:
+https://github.com/wxWidgets/wxWidgets/releases/download/v3.0.2/wxWidgets-3.0.2.tar.bz2 .
+
+Don't be tempted to use the Widgets version already installed on Linux because 
+this will cause problems.
+
+The libsndfile library is also required and is included in Audacity obtained 
+from GitHub. Obtain the latest Audacity alpha development source code from 
+https://github.com/audacity/audacity then use the green "Clone or download" 
+button to git clone Audacity. You may need to install git first.  
+
+If you are not planning to do any development, you may be better to download
+the Audacity 2.1.3 release minimal source tarball from 
+http://www.audacityteam.org/download/source/ .
+ 
+CMake ( http://www.cmake.org/ ) is required to build the local copy of 
+the libsoxr resampling library used by Audacity, unless you install 
+libsoxr-dev and use that system library.
+
+Installation of other libraries is optional, see:
+http://wiki.audacityteam.org/wiki/Developing_On_Linux#optional . 
+
+If you install libraries using a package management system like Apt or RPM,
+you need to install the "dev" (development) packages for each library.
+
+Compile wxWidgets with the default gtk2 option, not with gtk3:
+
+   ./configure
+   make
+
+Install wxWidgets (this defaults to installation in /usr/local/). 
+
+   make install # as root
+   ldconfig  # as root 
+
+The root command is often sudo on GNU/Linux, for example 'sudo make install'.
+
+Then to compile and install Audacity: 
+
+   ./configure CXXFLAGS="-std=gnu++11"
+   make
+   make install  # as root
+
+To see the compile-time options you can set, type
+   ./configure --help
+
+If you want to do any development, you might want to generate a configure cache
+and header dependencies:
+
+   ./configure -C
+   make dep
+
+You can ask for help with compilation problems at:
+http://forum.audacityteam.org/viewforum.php?f=19 .