Add snap packaging support

This commit adds a snapcraft.yaml file to allow lz4
to be distributed as a snap (see https://snapcraft.io/
for more info on snaps).

Building the Snap
-----------------

To build the snap simply install snapcraft and build the
snap e.g. on Ubuntu by doing:

$ sudo apt install snapcraft
$ cd contrib; snapcraft

This will build a snap that can be published [1] to
e.g. https://snapcraft.io/lz4

If you want to test the snap before publishing you
can do:

$ snap install <snapname>.snap --dangerous

And that will install it on your local host. You
will then have lz4 as a command to run.

Installing the Snap
-------------------

Once published and released in the snapstore your snap
can be installed on any platform that supports snaps
by doing:

snap install lz4

[1] https://docs.snapcraft.io/releasing-to-the-snap-store

contrib/snap/README.md

@@ -0,0 +1,29 @@
+Snap Packaging
+--------------
+
+This directory contains the config required to generate a snap package
+of lz4. Snaps are universal Linux packages that allow you to easily
+build your application from any source and ship it to any Linux
+distribution by publishing it to https://snapcraft.io/. A key attribute
+of a snap package is that it is (ideally) confined such that it
+executes within a controlled environmenti with all its dependencies
+bundled with it and does not share dependencies with of from any other
+package on the system (with a couple of minor exceptions).
+
+The basic anatomy and workflow is:
+
+  * ensure snap.snapcraft.yaml is up-to-date e.g. with version info
+
+  * build the snap by installing the snapcraft package and running it
+
+  * push snap/* changes to the repo (excluding any crud generated by a build of course)
+
+  * register yourself as owner of lz4 name in snapstore
+
+  * publish new snap to the snap store
+
+  * install snap by doing 'snap install lz4' on any Linux distro
+
+  * all installed copies of lz4 will be automatically updated to your new version
+
+For more information on Snaps see https://docs.snapcraft.io and https://forum.snapcraft.io/

contrib/snap/snapcraft.yaml

@@ -0,0 +1,31 @@
+name: lz4
+version: 1.8.4
+summary: Extremely Fast Compression algorithm 
+description: >
+    LZ4 is lossless compression algorithm, providing compression
+    speed > 500 MB/s per core, scalable with multi-cores CPU. It features an
+    extremely fast decoder, with speed in multiple GB/s per core, typically
+    reaching RAM speed limits on multi-core systems.
+    .
+    Speed can be tuned dynamically, selecting an "acceleration" factor which
+    trades compression ratio for faster speed. On the other end, a high
+    compression derivative, LZ4_HC, is also provided, trading CPU time for
+    improved compression ratio. All versions feature the same decompression
+    speed.
+    .
+    LZ4 is also compatible with dictionary compression, and can ingest any
+    input file as dictionary, including those created by Zstandard Dictionary
+    Builder. (note: only the final 64KB are used).
+    .
+    LZ4 library is provided as open-source software using BSD 2-Clause license.
+confinement: strict
+grade: stable
+
+apps:
+  lz4:
+    command: usr/local/bin/lz4
+    plugs: [home]
+parts:
+  lz4:
+    source: ../
+    plugin: make