readme: improve flow of the Installation section
Emphasize that go-ceph is a library and that the ceph devel packages are
build-time dependencies.
@@ -19,20 +19,25 @@ switch away from shelling out to the CLI and to these native function calls.
## Installation
- go get github.com/ceph/go-ceph
-
-The native RADOS library and development headers are expected to be installed.
+The code in go-ceph is purely a library module. Typically, one will import
+go-ceph in another Go based project. When building the code the native RADOS,
+RBD, & CephFS library and development headers are expected to be installed.
-On debian systems (apt):
+On debian based systems (apt) these may be:
```sh
libcephfs-dev librbd-dev librados-dev
```
-On rpm based systems (dnf, yum, etc):
+On rpm based systems (dnf, yum, etc) these may be:
```sh
libcephfs-devel librbd-devel librados-devel
```
+To quickly test if one can build with go-ceph on your system, run:
+```sh
+go get github.com/ceph/go-ceph
+```
+
go-ceph tries to support different Ceph versions. However some functions might
only be available in recent versions, and others may be deprecated. In order to
work with non-current versions of Ceph, it is required to pass build-tags to
