Update documentation for solidity-upgrade 0.7.0

docs/070-breaking-changes.rst

@@ -0,0 +1,17 @@
+********************************
+Solidity v0.7.0 Breaking Changes
+********************************
+
+This section highlights the main breaking changes introduced in Solidity
+version 0.7.0, along with the reasoning behind the changes and how to update
+affected code.
+For the full list check
+`the release changelog <https://github.com/ethereum/solidity/releases/tag/v0.7.0>`_.
+
+How to update your code
+=======================
+
+This section gives detailed instructions on how to update prior code for every breaking change.
+
+* Change ``f.value(...)()`` to ``f{value: ...}()``. Similarly ``(new C).value(...)()`` to
+  ``(new C){value: ...}()`` and ``f.gas(...)()`` to ``f{gas: ...}()``.