doc/rpm: Add information about new functionality

Signed-off-by: Michal Berger <michalx.berger@intel.com>
Change-Id: Ia89ca218681144147eb196cdec67b6e6c19a97b4
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/8546
Community-CI: Broadcom CI <spdk-ci.pdl@broadcom.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Paul Luse <paul.e.luse@intel.com>
Reviewed-by: Konrad Sztyber <konrad.sztyber@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>

doc/rpm.md

@@ -3,6 +3,7 @@
 ## In this document {#rpms_toc}
 
 * @ref building_rpms
+* @ref dpdk_devel
 
 ## Building SPDK RPMs {#building_rpms}
 
@@ -26,6 +27,13 @@ There are several options that may be passed via environment as well:
 - RPM_RELEASE   - Target release version of the RPM packages. Default: 1
 - REQUIREMENTS  - Extra set of RPM dependencies if deemed as needed
 - SPDK_VERSION  - SPDK version. Default: currently checked out tag
+- GEN_SPEC      - Orders rpm.sh to only generate a valid .spec and print
+                it on stdout. The content of the .spec is determined based
+                mainly on the ./configure cmdline passed to rpm.sh.
+- USE_DEFAULT_DIRS - Normally, rpm.sh will order rpmbuild to build under
+                   customizable set of directories. Since this may be not
+                   desired, especially when used together with GEN_SPEC,
+                   this option will preserve the default set of directories.
 
 ~~~{.sh}
 # DEPS=no MAKEFLAGS="-d -j1" rpmbuild/rpm.sh --with-shared
@@ -47,3 +55,13 @@ target user:
 - spdk-devel      - provides development files
 - spdk-libs       - provides target lib, .pc files (--with-shared)
 - spdk-dpdk-libs  - provides dpdk lib files (--with-shared|--with-dpdk)
+
+## Special case for dpdk-devel {#dpdk_devel}
+
+When rpm.sh finds a bare --with-dpdk argument on the cmdline it will try to
+adjust the behavior of the rpmbuild to make sure only SPDK RPMs are built.
+Since this argument requests SPDK to be built against installed DPDK (e.g.
+dpdk-devel package) the spdk-dpdk-libs RPM won't be included.  Moreover, the
+.spec will be armed with a build requirement to make sure dpdk-devel is
+present on the building system. The minimum required version of dpdk-devel
+is set to 19.11.