Added missing system requiremnt and pip upgrade before install

When installing on CentOS7 I wans't able to follow README instructions to
install due to errors. I was missing libsodium in order to compile
python dependencies.

Default version of Python pip is really old and therefore setuptools
upgrade ended with error as well. In order to be able to continue I
needed to upgrade pip as well.

README.rst

@@ -111,7 +111,7 @@ Installing prerequisites on ArchLinux::
 Installing prerequisites on CentOS 7::
 
     sudo yum install libtiff-devel libjpeg-devel libzip-devel freetype-devel \
-                     lcms2-devel libwebp-devel tcl-devel tk-devel \
+                     lcms2-devel libwebp-devel libsodium tcl-devel tk-devel \
                      python-virtualenv libffi-devel openssl-devel
     sudo yum groupinstall "Development Tools"
 
@@ -146,6 +146,7 @@ To install the synapse homeserver run::
 
     virtualenv -p python2.7 ~/.synapse
     source ~/.synapse/bin/activate
+    pip install --upgrade pip
     pip install --upgrade setuptools
     pip install https://github.com/matrix-org/synapse/tarball/master