Document the version checking

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@@ -26,6 +26,7 @@ Contents
    srp
    rng
    fpe
+   versions
    python
    relnotes/index
 
diff --git a/doc/versions.rst b/doc/versions.rst
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+
+Version Checking
+========================================
+
+The library has functions for checking compile-time and runtime
+versions.
+
+All versions are of the tuple (major,minor,patch). Even minor versions
+indicate stable releases while odd minor versions indicate a
+development release.
+
+The compile time version information is defined in `botan/build.h`
+
+.. c:macro:: BOTAN_VERSION_MAJOR
+
+   The major version of the release.
+
+.. c:macro:: BOTAN_VERSION_MINOR
+
+   The minor version of the release.
+
+.. c:macro:: BOTAN_VERSION_PATCH
+
+   The major version of the release.
+
+.. c:macro:: BOTAN_VERSION_DATESTAMP
+
+   Expands to an integer of the form YYYYMMDD if this is an official
+   release, or 0 otherwise. For instance, 1.10.1, which was released
+   on July 11, 2011, has a `BOTAN_VERSION_DATESTAMP` of 20110711.
+
+.. c:macro:: BOTAN_DISTRIBUTION_INFO
+
+   .. versionadded:: 1.9.3
+
+   A macro expanding to a string that is set at build time using the
+   ``--distribution-info`` option. It allows a packager of the library
+   to specify any distribution-specific patches. If no value is given
+   at build time, the value is 'unspecified'.
+
+.. c:macro:: BOTAN_VERSION_VC_REVISION
+
+   .. versionadded:: 1.10.1
+
+   A macro expanding to a string that is set to a revision identifier
+   cooresponding to the source, or 'unknown' if this could not be
+   determined. It is set for all official releases and for releases
+   occuring within a Monotone workspace.
+
+The runtime version information, and some helpers for compile time
+version checks, are included in `botan/version.h`
+
+.. cpp:function:: std::string version_string()
+
+   Returns a single-line string containing relevant information about
+   this build and version of the library in an unspecified format.
+
+.. cpp:function:: u32bit version_major()
+
+   Returns the major part of the version.
+
+.. cpp:function:: u32bit version_minor()
+
+   Returns the minor part of the version.
+
+.. cpp:function:: u32bit version_patch()
+
+   Returns the patch part of the version.
+
+.. cpp:function:: u32bit version_datestamp()
+
+   Return the datestamp of the release (or 0 if the current version is
+   not an official release).
+
+.. c:macro:: BOTAN_VERSION_CODE_FOR(maj,min,patch)
+
+   Return a value that can be used to compare versions. The current
+   (compile-time) version is available as the macro
+   `BOTAN_VERSION_CODE`. For instance, to choose one code path for
+   versions before 1.10 and another for 1.10 or later::
+
+      #if BOTAN_VERSION_CODE >= BOTAN_VERSION_CODE_FOR(1,10,0)
+         // 1.10 code path
+      #else
+         // pre-1.10 code path
+      #endif
+