doc: how to set version number in build (#350)

* version-number-in-build

* Update README.md

README.md

@@ -24,7 +24,7 @@ See https://pekko.apache.org for the documentation including the API docs. The d
 ### Running the Build
 - Open a command window and change directory to your preferred base directory
 - Use git to clone the [repo](https://github.com/apache/incubator-pekko) or download a source release from https://pekko.apache.org (and unzip or untar it, as appropriate)
-- Change directory to the directory where you installed the source
+- Change directory to the directory where you installed the source (you should have a file called `build.sbt` in this directory)
 - `sbt compile` compiles the main source for project default version of Scala (2.13)
     - `sbt +compile` will compile for all supported versions of Scala
 - `sbt test` will compile the code and run the unit tests
@@ -39,6 +39,12 @@ See https://pekko.apache.org for the documentation including the API docs. The d
      - the `index.html` file will appear in `target/paradox/site/main/`
 - `sbt unidoc` will build the Javadocs for all the modules and load them to one place (may require Graphviz, see Prerequisites above)
      - the `index.html` file will appear in `target/scala-2.13/unidoc/`
+- `sbt sourceDistGenerate` will generate source release to `target/dist/`
+- The version number that appears in filenames and docs is derived, by default. The derived version contains the most git commit id or the date/time (if the directory is not under git control). 
+    - You can set the version number explicitly when running sbt commands
+        - eg `sbt "set ThisBuild / version := \"1.0.0\"; sourceDistGenerate"`  
+    - Or you can add a file called `version.sbt` to the same directory that has the `build.sbt` containing something like
+        - `ThisBuild / version := "1.0.0"` 
 
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