[librsvg] COMPILING.md: Note on how to run `make distcheck` so it will work

[Federico Mena Quintero <federico src gnome org>]

commit aa9e0ba1ec2fcdb258f5bb4c8b3fb9b49dfe32a7
Author: Federico Mena Quintero <federico gnome org>
Date:   Fri Jun 1 08:43:57 2018 -0500

    COMPILING.md: Note on how to run `make distcheck` so it will work

 COMPILING.md | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)
---
diff --git a/COMPILING.md b/COMPILING.md
index a8c517c4..b7f29e56 100644
--- a/COMPILING.md
+++ b/COMPILING.md
@@ -21,6 +21,7 @@ explains librsvg's peculiarities.
 * [Debug or release builds](#debug-or-release-builds)
 * [Cross-compilation](#cross-compilation)
 * [Building with no network access](#building-with-no-network-access)
+* [Running `make distcheck`](#running-make-distcheck)
 
 # Installing dependencies for building
 
@@ -241,6 +242,22 @@ one of the Rust crates that librsvg uses internally.
 The source replacement information is in `rust/.cargo/config` in the
 unpacked tarball.  Your build system can patch this file as needed.
 
+# Running `make distcheck`
+
+The `make distcheck` command will built a release tarball, extract it,
+compile it and test it.  However, part of the `make install` process
+within that command will try to install the gdk-pixbuf loader in your
+system location, and it will fail.
+
+Please run `make distcheck` like this:
+
+```
+$ make distcheck DESTDIR=/tmp/foo
+```
+
+That `DESTDIR` will keep the gdk-pixbuf loader installation from
+trying to modify your system locations.
+
 [autotools]: https://autotools.io/index.html
 [blog]: https://people.gnome.org/~federico/blog/librsvg-build-infrastructure.html
 [maintainer]: README.md#maintainers