Documentation changes.

docs/introduction-to-smart-contracts.rst

@@ -31,8 +31,12 @@ Storage Example
         }
     }
 
-The first line tells you that the source code is written for
-Solidity version 0.4.0, or a newer version of the language up to, but not including version 0.7.0.
+The first line tells you that the source code is licensed under the
+GPL version 3.0. Machine-readable license specifiers are important
+in a setting where publishing the source code is the default.
+
+The next line specifies that the source code is written for
+Solidity version 0.4.16, or a newer version of the language up to, but not including version 0.7.0.
 This is to ensure that the contract is not compilable with a new (breaking) compiler version, where it could behave differently.
 :ref:`Pragmas<pragma>` are common instructions for compilers about how to treat the
 source code (e.g. `pragma once <https://en.wikipedia.org/wiki/Pragma_once>`_).

docs/layout-of-source-files.rst

@@ -7,6 +7,38 @@ Source files can contain an arbitrary number of
 :ref:`pragma directives<pragma>` and
 :ref:`struct<structs>` and :ref:`enum<enums>` definitions.
 
+.. index:: ! license, spdx
+
+SPDX License Identifier
+=======================
+
+Trust in smart contract can be better established if their source code
+is available. Since making source code available always touches on legal problems
+with regards to copyright, the Solidity compiler encouranges the use
+of machine-readable `SPDX license identifiers <https://spdx.org>`_.
+Every source file should start with a comment indicating its license:
+
+``// SPDX-License-Identifier: MIT``
+
+The compiler does not validate that the license is part of the
+`list allowed by SPDX <https://spdx.org/licenses/>`_, but
+it does include the supplied string in the `bytecode metadata <metadata>`_.
+
+If you do not want to specify a license or if the source code is
+not open-source, please use the special value ``UNLICENSED``.
+
+Supplying this comment of course does not free you from other
+obligations related to licensing like having to mention
+a specific license header in each source file or the
+original copyright holder.
+
+The comment is recognized by the compiler anywhere in the file at the
+file level, but it is recommended to put it at the top of the file.
+
+More information about how to use SPDX license identifiers
+can be found at the `SPDX website <https://spdx.org/ids-how>`_.
+
+
 .. index:: ! pragma
 
 .. _pragma:

docs/metadata.rst

@@ -1,3 +1,5 @@
+.. _metadata:
+
 #################
 Contract Metadata
 #################
@@ -54,7 +56,9 @@ explanatory purposes.
           // Required (unless "content" is used, see below): Sorted URL(s)
           // to the source file, protocol is more or less arbitrary, but a
           // Swarm URL is recommended
-          "urls": [ "bzzr://56ab..." ]
+          "urls": [ "bzzr://56ab..." ],
+          // Optional: SPDX license identifier as given in the source file
+          "license": "MIT"
         },
         "destructible": {
           // Required: keccak256 hash of the source file