Streamline DX for Ubuntu 20 (#69)

* Add documentation of what packages to install for Ubuntu build.

* Harmonize Readme markup.

.travis.yml

@@ -1,7 +1,7 @@
 language: c++
 git:
   depth: 3
-dist: xenial
+dist: focal
 cache: pip
 
 before_install:

README.md

@@ -41,11 +41,9 @@ After cloning the repository, add git submodules
 
 FreeSASA can be compiled and installed using the following
 
-```sh
-    autoreconf -i  # only if cloned from the git-repo
+    autoreconf -i # only necessary if you're cloning git repo
     ./configure
     make && make install
-```    
 
 NB: If the source is downloaded from the git repository the
 configure-script needs to be set up first using `autoreconf -i`. Users
@@ -140,6 +138,18 @@ It should be relatively straightforwad to build the command line tool for
 Windows using MinGW or Cygwin, but this hasn't been tested (let me know if
 you've got it to work).
 
+### Prerequisites for Ubuntu
+
+The following command will install all dependencies needed, some of which most users will already have,
+for a minimal build of FreeSASA in Ubuntu (verified for version 16, 18 and 20).
+
+    apt-get update
+    apt-get install git build-essential autoconf libc++-dev libc++abi-dev
+
+For a fully featured build, with ability to run unit tests, these additional packages are needed
+
+    apt-get install check libjson-c-dev libxml2-dev libxml2-utils
+
 ## Citing FreeSASA
 
 FreeSASA can be cited using the following publication