[librsvg: 5/9] Compiling: Add note about building on ubunut and using rustup.

[Federico Mena Quintero <federico src gnome org>]

commit 7697aa9c93b0002c97f6fda5c690f745670e14a8
Author: Jordan Petridis <jordanpetridis protonmail com>
Date:   Mon Feb 19 11:27:37 2018 +0000

    Compiling: Add note about building on ubunut and using rustup.

 COMPILING.md | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)
---
diff --git a/COMPILING.md b/COMPILING.md
index 5b70ade8..e66eedcd 100644
--- a/COMPILING.md
+++ b/COMPILING.md
@@ -23,15 +23,20 @@ explains librsvg's peculiarities.
 # Building Enviroment Depenancies:
 
 * Gtk+ >= 3.10
-* Rust >= "Compiler version here"
+* Rust >= "1.21"
 
-### Debian/Ubuntu based systems:
+### Ubuntu based systems:
+librsvg can not be built in debian stable due to and old version of `glib`.
 librsvg can not be built in ubuntu 16.04, due to some of the dependancies provided being old.
 
-The following was only tested on ubuntu 18.04, plese provide feedback if it works on other platforms.
+**Building on 18.04:**
+
+Currently you will need to install `rustc` and `cargo` with [rustup.rs](https://rustup.rs)
+due to the `rustc` package provided from the ubuntu repo being old. This should not be the case
+once 18.04 is release.
 
 ```sh
-apt install -y gcc rustc cargo make \
+apt install -y gcc make \
 automake autoconf libtool gettext itstool \
 libgdk-pixbuf2.0-dev libgirepository1.0-dev \
 gtk-doc-tools git  libgtk-3-dev \