Write contrinbuting guidelines

.gitignore

@@ -9,3 +9,6 @@
 cabal.sandbox.config
 cabal.project.local
 /graphql.cabal
+
+# GHC
+*.hi

CHANGELOG.md

@@ -17,6 +17,7 @@ and this project adheres to
 - Type system definition parser.
 - `Trans.argument`.
 - Schema extension parser.
+- Contributing guidelines
 
 ### Changed
 - Rename `AST.Definition` into `AST.Document.ExecutableDefinition`.

CONTRIBUTING.md

@@ -0,0 +1,31 @@
+# Contributing guidelines
+
+## Testing
+
+To ensure all code changes adhere to existing code quality standards, some
+automatic checks can be run locally.
+
+Ensure that the code builds without warnings and passes the tests:
+
+```sh
+stack test --pedantic
+```
+
+And also run the linter on your code:
+
+```sh
+stack build hlint
+stack exec hlint -- src tests
+```
+
+Build the documentation and check if you get any warnings:
+
+```sh
+stack haddock
+```
+
+Validate that literate Haskell (tutorials) files compile without any warnings:
+
+```sh
+stack ghc -- -Wall -fno-code docs/tutorial/*.lhs
+```

README.md

@@ -24,11 +24,18 @@ For the list of currently missing features see issues marked as
 ## Documentation
 
 API documentation is available through
-[hackage](https://hackage.haskell.org/package/graphql).
+[Hackage](https://hackage.haskell.org/package/graphql).
 
 You'll also find a small tutorial with some examples under
 [docs/tutorial](https://github.com/caraus-ecms/graphql/tree/master/docs/tutorial).
 
+## Further information
+
+- [Contributing guidelines](CONTRIBUTING.md).
+- [Changelog](CHANGELOG.md) – this one contains the most recent changes; 
+  individual changelogs for specific versions can be found on
+  [Hackage](https://hackage.haskell.org/package/graphql).
+
 ## Contact
 
 Suggestions, contributions and bug reports are welcome.

stack.yaml

@@ -1,4 +1,4 @@
-resolver: lts-15.0
+resolver: lts-15.7
 
 packages:
 - .