Add instructions in README to convert to SPDX 3.

Signed-off-by: Marc-Etienne Vargenau <[email protected]>

README.md

@@ -45,7 +45,7 @@ The following converter tools support spdx format:
   * JSON
   * XML
   * YAML
-  * JSON-LD (SPDX spec version 3.0)
+  * JSON-LD (SPDX spec version 3.0.1)
 
 Example to convert a SPDX file from tag to rdf format:
 
@@ -55,6 +55,13 @@ The file formats can optionally be provided as the 3rd and 4th parameter for the
 
     java -jar tools-java-2.0.0-Alpha-jar-with-dependencies.jar Convert ../testResources/SPDXTagExample-v2.2.spdx TagToRDF.ttl TAG RDFTTL excludeLicenseDetails
 
+To convert from SPDX 2 to SPDX 3.0.1:
+* use the file extension `.jsonld.json` or `.jsonld`;
+* or add the options for the from and to file types:
+```
+java -jar tools-java-2.0.0-Alpha-jar-with-dependencies.jar Convert hello.spdx hello.spdx.json TAG JSONLD
+```
+
 ## Compare utilities
 The following  tools can be used to compare one or more SPDX documents: