Update contributing section (#154)

README.md

@@ -29,7 +29,52 @@ This command will iterate over all exercises and check to see if their exemplar/
 Please see [Exercism's contributing guide](https://exercism.org/docs/building).
 
 At the moment, there's not a generator for Pyret exercises.
-Make sure `use context essentials2020` is the first line in all Pyret files to explicitly set the default namespace.
+
+Here's the basic template for an `exercise-slug-test`.arr.
+Each `check` block corresponds to a single test case, and the string label is reported to the student.
+Each `check` block is wrapped inside a no-parameter function which is then stored inside the `run` field of a `test` value of the `TestRun` datatype.
+This `test` value also contains an `active` field which indicates whether a test should be run (`true`) or not (`false`).
+All `test` values go inside a list that Pyret iterates over at runtime, executing the functions within each `test` value marked as active.
+
+A contributor is responsible for copying this template, adding the appropriate functions and `check` blocks, and populating the list at the bottom.
+
+```pyret
+
+use context essentials2020
+
+include file("exercise-slug.arr")
+
+#|
+  When working offline, all tests except the first one are skipped by default.
+  Once you get the first test running, unskip the next one until all tests pass locally.
+  Check the block comment below for further details.
+|#
+
+fun foo-returns-1():
+  check "foo returns 1":
+    foo() is 1
+  end
+end
+
+fun bar-returns-2():
+  check "bar returns 2":
+    bar() is 2
+  end
+end
+
+#|
+  Code to run each test. Each line corresponds to a test above and whether it should be run.
+  To mark a test to be run, replace `false` with `true` on that same line after the comma.
+  test(test-a, true) will be run. test(test-a, false) will be skipped.
+|#
+
+data TestRun: test(run, active) end
+
+[list: 
+  test(foo true),
+  test(bar, false),
+].each(lam(t): when t.active: t.run() end end)
+```
 
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