Do you have the readme file included in the project.wms file?
<Fragment>
<ComponentGroup Id="ProductComponents" Directory="INSTALLFOLDER">
<Component Id="README.txt">
<File Source="README.txt" Id="README.txt" />
</Component>
</ComponentGroup>
<Fragment>
I also used WixShellExecTarget to open the file, that means the user will have it open in whatever text editor they have as default.