platform specific prerequisites in source install

INSTALL.md

@@ -6,7 +6,7 @@ There are 3 steps to follow under **Installation Instructions**.
   - [Choosing your server name](#choosing-your-server-name)
   - [Installing Synapse](#installing-synapse)
     - [Installing from source](#installing-from-source)
-      - [Platform-Specific Instructions](#platform-specific-instructions)
+      - [Platform-specific prerequisites](#platform-specific-prerequisites)
         - [Debian/Ubuntu/Raspbian](#debianubunturaspbian)
         - [ArchLinux](#archlinux)
         - [CentOS/Fedora](#centosfedora)
@@ -60,17 +60,14 @@ that your email address is probably `user@example.com` rather than
 
 (Prebuilt packages are available for some platforms - see [Prebuilt packages](#prebuilt-packages).)
 
+When installing from source please make sure that the [Platform-specific prerequisites](#platform-specific-prerequisites) are already installed.
+
 System requirements:
 
 - POSIX-compliant system (tested on Linux & OS X)
 - Python 3.5.2 or later, up to Python 3.9.
 - At least 1GB of free RAM if you want to join large public rooms like #matrix:matrix.org
 
-Synapse is written in Python but some of the libraries it uses are written in
-C. So before we can install Synapse itself we need a working C compiler and the
-header files for Python C extensions. See [Platform-Specific
-Instructions](#platform-specific-instructions) for information on installing
-these on various platforms.
 
 To install the Synapse homeserver run:
 
@@ -128,7 +125,11 @@ source env/bin/activate
 synctl start
 ```
 
-#### Platform-Specific Instructions
+#### Platform-specific prerequisites
+
+Synapse is written in Python but some of the libraries it uses are written in
+C. So before we can install Synapse itself we need a working C compiler and the
+header files for Python C extensions.
 
 ##### Debian/Ubuntu/Raspbian
 

changelog.d/9667.doc

@@ -0,0 +1 @@
+Update source install documentation to mention platform prerequisites before the source install steps.