libxml2 should be installed before running carton

[smonff]

Following my report #13, I think the towncrier documentation could mention that it's installation require libxml2 and libxml2-dev to be installed from the OS packages or something, before running carton install.

If not, some related packages won't install properly:

smonff@fopux:~/dev/towncrier$ carton install                    
Installing modules using /home/sfeu/dev/rd/towncrier/cpanfile                                                          
! Configure failed for XML-LibXML-2.0132. See /home/sfeu/.cpanm/work/1546441529.48609/build.log for details.  
! Installing the dependencies failed: Module 'XML::LibXML' is not installed                                   
! Bailing out the installation for XML-Atom-0.42.                                                                       
! Installing the dependencies failed: Module 'XML::LibXML' is not installed, Module 'XML::Atom' is not installed    
! Bailing out the installation for XML-Feed-0.55.                                                            
! Installing the dependencies failed: Module 'XML::Feed' is not installed                                                  
! Bailing out the installation for Dancer-Plugin-Feed-1.131470.                                               
! Installing the dependencies failed: Module 'Dancer::Plugin::Feed' is not installed, Module 'XML::LibXML' is not installed
! Bailing out the installation for /home/sfeu/dev/rd/towncrier/.                                                                 
Installing modules failed       

Hope it helps.