Update docs (#107)

Author: angerman

changelog.md

@@ -0,0 +1,3 @@
+## Mar 15, 2019
+  * overlays was renamed to extras (#79)  
+    See docs/migration.md

docs/iohk-nix.md

@@ -52,14 +52,6 @@ let
   # our packages
   stack-pkgs = import ./.stack-pkgs.nix;
 
-  # packages which will require TH and thus
-  # will need -fexternal-interpreter treatment
-  # when cross compiling.
-  th-packages = [
-    "hedgehog" "cardano-crypto-wrapper"
-    "cardano-crypto-test" "cardano-chain"
-    "small-steps" "cs-ledger" ];
-
   # Build the packageset with module support.
   # We can essentially override anything in the modules
   # section.
@@ -93,8 +85,7 @@ let
       # work when cross compiling.  For now we need to
       # list the packages that require template haskell
       # explicity here.
-      (iohk-module { nixpkgs = pkgs;
-                     inherit th-packages; })
+      iohk-module
     ];
   };
 in

docs/migrations.md

@@ -0,0 +1,13 @@
+# `overlay` to `extras`
+
+`overlay` was renamed to `extras` in #79 to prevent confusion between the notion of nix overlays.
+Therefore `plan-pkgs` and `stack-pkgs` as generated by `plan-to-nix` and `stack-to-nix` will
+expose `extras` instead of `overlay`. Similarly `mkStackPkgSet`, `mkPkgSet` and `mkCabalProjectPkgSet`
+take a `pkg-def-extras` instead of `pkg-def-overlay` argument.  If you are using `iohk-nix`, the
+`iohk-overlay` was parameter was renamed to `iohk-extras`.
+
+# Drop `th-packages`
+
+When using `iohk-nix` to facility cross compilation, we used to require explicit listing of packages
+that required `TemplateHaskell`, this was dropped in [iohk-nix@da7c1ed](https://github.com/input-output-hk/iohk-nix/commit/da7c1edc31d24ff4a211b7dc2361c71af7c3bda4), see #92.
+As such the `th-packages` and `nixpkgs` argument were dropped from the `iohk-module`.