Merge pull request bitcoin#1918 from murchandamus/bip3-reformat-existing-licenses

BIP3: Update License header format on activation

Author: jonatack

bip-0003.md

@@ -588,7 +588,8 @@ The Superseded-By header is replaced with the Proposed-Replacement header in all
 
 #### Licenses
 
-The licenses of existing BIPs remain untouched.
+Existing BIPs retain their license terms unchanged.
+The License and License-Code headers of BIPs are updated to express those terms using SPDX License Expressions.
 
 ## Copyright