Document and update cdt.tests.dsf.gdb.versions build variable

Change-Id: I8ec4216a875f8c23fb7c4594c6414a23bac0a46d

README.md

@@ -121,6 +121,16 @@ For running CDT's DSF-GDB tests, this specifies the path to the location of gdb.
 
 For running CDT's DSF-GDB tests, this specifies the executable names of the gdbs to run, comma-separated.
 
+There are a few special values that can be specified (see BaseParametrizedTestCase for source):
+
+- all: run all versions listed in ITestConstants.ALL_KNOWN_VERSIONS
+- supported: run all versions listed in ITestConstants.ALL_SUPPORTED_VERSIONS
+- unsupported: run all versions listed in ITestConstants.ALL_UNSUPPORTED_VERSIONS
+
+The default, defined in the root pom.xml, should be the most recent released version of gdb.
+
+To build all gdb versions for testing CDT see [download-build-gdb.sh](https://github.com/eclipse-cdt/cdt-infra/blob/master/docker/scripts/download-build-gdb.sh)
+
 #### native
 
 The `native` property can be used to build the native libraries. Defining the `native` property will activate profiles to add the extra steps to compile the natives libraries used by CDT. The main CDT build by default will not build the libraries, but instead use the versions of the libraries checked into git. Therefore when users modify the sources of the native libraries, they have to build and commit the changed library binaries as part of the commit.

pom.xml

@@ -74,7 +74,7 @@
 		     users expect. -->
 		<skipTests>${cdt-other.skip.tests}</skipTests>
 		<dsf.gdb.tests.gdbPath>/shared/common/gdb/gdb-all/bin</dsf.gdb.tests.gdbPath>
-		<cdt.tests.dsf.gdb.versions>gdb.8.3</cdt.tests.dsf.gdb.versions>
+		<cdt.tests.dsf.gdb.versions>gdb.10.1</cdt.tests.dsf.gdb.versions>
 		<maven.build.timestamp.format>yyyyMMdd-HHmm</maven.build.timestamp.format>
 		<buildTimestamp>${maven.build.timestamp}</buildTimestamp>
 	    <buildId>${buildTimestamp}</buildId>