compression: added docs and changelog info on reduce/compression bdev

Change-Id: Ib4bd9759bb97080b5ccc560580ac4519f105c8a6
Signed-off-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Signed-off-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/452652
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>

CHANGELOG.md

@@ -192,6 +192,12 @@ very large performance improvements, especially on GCC 8 and clang 7. This may
 be combined with link time optimization which has been available under the
 `--enable-lto` configure option for several releases.
 
+### compression bdev/reduce library
+
+Added "reduce" block compression scheme based on using SSDs for storing
+compressed blocks of storage and presistent memory for metadata. Please see
+[compression](https://spdk.io/doc/bdev.html) for more details.
+
 ## v19.01:
 
 ### ocf bdev

doc/Doxyfile

@@ -808,6 +808,7 @@ INPUT                  += \
                          blob.md \
                          blobfs.md \
                          changelog.md \
+                         compression.md \
                          concurrency.md \
                          directory_structure.md \
                          event.md \

doc/prog_guides.md

@@ -7,3 +7,4 @@
 - @subpage nvmf_tgt_pg
 - @subpage ftl
 - @subpage gdb_macros
+- @subpage reduce