Docs: add a README, explain high-level usage

README.md

@@ -0,0 +1,72 @@
+# KPMcore
+
+> KPMcore, the KDE Partition Manager core, is a library for examining
+> and modifying partitions, disk devices, and filesystems on a
+> Linux system. It provides a unified programming interface over
+> top of (external) system-manipulation tools.
+
+KPMcore is a library for examining and manipulating all facets
+of storage devices on a system:
+* raw disk devices
+* partition tables on a device
+* filesystems within a partition
+
+There are multiple backends so that KPMcore can support different
+operating systems, although the only functional backend is the
+one for Linux systems:
+* libparted backend (Linux)
+* null backend
+
+## Using KPMcore
+
+Most of the usage information on KPMcore is included in the API
+documentation; this section contains only high-level usage information.
+
+### Finding KPMcore with CMake
+
+KPMcore supports CMake as (meta-)build system and installs suitable
+CMake support files. Typical use of of KPMcore in a `CMakeLists.txt`
+looks like this:
+
+```
+    find_package( KPMcore 3.2 REQUIRED )
+    include_directories( ${KPMCORE_INCLUDE_DIR} )
+    target_link_libraries( target kpmcore )
+```
+
+There are no imported targets defined for KPMcore.
+
+### Initialization
+
+An application must initialize the library and load a suitable
+backend before using KPMcore functions. By convention, the
+environment variable `KPMCORE_BACKEND` names a backend,
+and typical initialization code will look like this (or use the
+class `KPMCoreInitializer` from `test/helpers.h`):
+
+```
+    #include <kpmcore/backend/corebackendmanager.h>
+    #include <QDebug>
+
+    bool initKPMcore()
+    {
+        static bool inited = false;
+        if ( inited ) return true;
+
+        QByteArray env = qgetenv( "KPMCORE_BACKEND" );
+        auto backendName = env.isEmpty() ? CoreBackendManager::defaultBackendName() : env;
+        if ( !CoreBackendManager::self()->load( backendName )
+        {
+            qWarning() << "Failed to load backend plugin" << backendName;
+            return false;
+        }
+        inited = true;
+        return true;
+    }
+```
+
+This code uses the environment variable if set, and otherwise falls
+back to a default backend suitable for the current platform.
+
+Calling KPMcore functions before the library is initialized will
+result in undefined behavior.