release checklist: add versioning guide line

Author: wkliao

darshan-test/RELEASE-CHECKLIST.txt

@@ -13,6 +13,7 @@ Notes on how to release a new version of Darshan
  4) update the Changelog, if needed (browse git log since last release)
     - commit
  5) update version number in top-level darshan.version file
+    - See below for the guide line of setting the version number.
     - commit
  6) follow checklist for corresponding release of PyDarshan
     - found at darshan-util/pydarshan/RELEASE-CHECKLIST-PyDarshan.txt
@@ -49,3 +50,43 @@ Notes on how to release a new version of Darshan
       enabled by the release
     - commit, submit PR to upstream Spack repository
 14) announce on web page (new post) and mailing list
+
+----
+## Setting the release version number
+
+There are two versions that must be set when making a new release:
+software semantic version and libtool ABI version.
+
+* Software Semantic Versioning
+  + https://semver.org
+  + A version number is in a form of MAJOR.MINOR.PATCH
+    1. MAJOR version when you make incompatible API changes
+    2. MINOR version when you add functionality in a backward compatible manner
+    3. PATCH version when you make backward compatible bug fixes
+  + A pre-release version MAY be denoted by appending a hyphen, for example,
+    1.0.0-alpha.
+
+* GNU Libtool's ABI versioning
+  + http://www.gnu.org/software/libtool/manual/libtool.html#Updating-version-info
+  + A version number is in a form of current.revision.age
+    1. "current (c)" is the most recent interface number that this library
+       implements. A change in this number typically indicates a
+       backward-incompatible ABI change.
+    2. "revision (r)" is the implementation number of the current interface.
+       Tracks changes in the library's source code without affecting the ABI.
+    3. "age (a)" is the difference between the newest and oldest interfaces that
+       this library implements. It indicates the number of interfaces added
+       since the last current increment.
+  + Rules for Updating Version Information:
+   * Initial State: Begin with 0:0:0 for new libraries.
+   * Release-based Updates: Update versions only before a public release.
+   * Source Code Changes: Increment "revision" if the source code has changed
+     since the last update, even without interface changes.
+     (c:r:a becomes c:r+1:a)
+   * Interface Changes (Addition/Removal/Modification): Increment "current" and
+     reset "revision" to 0 if any interfaces have been added, removed, or changed.
+   * Interface Additions: Increment "age" if new interfaces have been added since
+     the last public release.
+   * Interface Removals: Reset "age" to 0 if any interfaces have been removed
+     since the last public release.
+