docs: use zlib,6 compression in some examples, fixes #275

This is what attic used by default, but borgbackup defaults to "no compression".

I just adjusted the command invocation, so we can keep the example output
(which shows that stuff was compressed).

Also: add FAQ item about compression.

README.rst

@@ -90,7 +90,7 @@ Initialize a new backup repository and create a backup archive::
 
 Now doing another backup, just to show off the great deduplication::
 
-    $ borg create --stats /mnt/backup::Tuesday ~/Documents
+    $ borg create --stats -C zlib,6 /mnt/backup::Tuesday ~/Documents
 
     Archive name: Tuesday
     Archive fingerprint: 387a5e3f9b0e792e91c...

docs/faq.rst

@@ -50,6 +50,17 @@ Which file types, attributes, etc. are *not* preserved?
       Archive extraction has optional support to extract all-zero chunks as
       holes in a sparse file.
 
+Why is my backup bigger than with attic?
+Why doesn't |project_name| do compression by default?
+    * attic was rather unflexible when it comes to compression, it always
+      compressed using zlib level 6 (no way to switch compression off or
+      adjust the level or algorithm)
+    * |project_name| offers a lot of different compression algorithms and
+      levels. Which of them is the best for you pretty much depends on your use
+      case, your data, your hardware - so you need to do an informed decision
+      about whether you want to use compression, which algorithm and which
+      level you want to use. This is why compression defaults to none.
+
 How can I specify the encryption passphrase programmatically?
     The encryption passphrase can be specified programmatically using the
     `BORG_PASSPHRASE` environment variable. This is convenient when setting up

docs/usage.rst

@@ -201,8 +201,9 @@ Examples
         --exclude '*.pyc'
 
     # Backup the root filesystem into an archive named "root-YYYY-MM-DD"
+    # use zlib compression (good, but slow) - default is no compression
     NAME="root-`date +%Y-%m-%d`"
-    $ borg create /mnt/backup::$NAME / --do-not-cross-mountpoints
+    $ borg create -C zlib,6 /mnt/backup::$NAME / --do-not-cross-mountpoints
 
     # Backup huge files with little chunk management overhead
     $ borg create --chunker-params 19,23,21,4095 /mnt/backup::VMs /srv/VMs
@@ -315,7 +316,7 @@ Examples
     Hostname: myhostname
     Username: root
     Time: Fri Aug  2 15:18:17 2013
-    Command line: /usr/bin/borg create --stats /mnt/backup::root-2013-08-02 / --do-not-cross-mountpoints
+    Command line: /usr/bin/borg create --stats -C zlib,6 /mnt/backup::root-2013-08-02 / --do-not-cross-mountpoints
     Number of files: 147429
     Original size: 5344169493 (4.98 GB)
     Compressed size: 1748189642 (1.63 GB)