docs: Improve linux installation instructions (#1566)

* Add missing crossbar linux deps to docs

* Add build depends for centos as well

* Suggest crossbar-pypy snap as well

Author: om26er

docs/Installation-on-Linux.rst

@@ -21,8 +21,8 @@ directory) and does not depend on system Python packages.
 Prepare
 -------
 
-Debian
-~~~~~~
+Debian/Ubuntu
+~~~~~~~~~~~~~
 
 After a fresh install of Ubuntu 16.04 LTS 64-bit Server, first make sure
 your system packages are fully up to date:
@@ -37,15 +37,17 @@ Then install the prerequisites:
 ::
 
     sudo apt-get -y install build-essential libssl-dev libffi-dev \
-       libreadline-dev libbz2-dev libsqlite3-dev libncurses5-dev
+        libreadline-dev libbz2-dev libsqlite3-dev libncurses5-dev \
+        libsnappy-dev libunwind-dev
 
 Snap
 ~~~~
 To install crossbar from snap use below command
 ::
 
-    $snap install crossbar
+    snap install crossbar
 
+.. note:: For our PyPy variant, install crossbar-pypy instead.
 .. note:: Snaps are strictly confined to their environment, so is it not possible to access arbitrary binaries on your system. Because of it some functionalities of Crossbar may not work.
 
 CentOS 7
@@ -62,7 +64,8 @@ Then install the prerequisites:
 
 ::
 
-    yum install gcc gcc-c++ make openssl-devel libffi-devel
+    yum install gcc gcc-c++ make openssl-devel libffi-devel \
+        libsnappy-devel libunwind-devel
 
 Now continue to build for: