README: move accumulate under practice

Should have been done with 2ef2c03,
which was a part of #936

This commit is part of #1083

Author: pwadsworth

README.md

@@ -94,23 +94,24 @@ The [track anatomy documentation](https://github.com/exercism/docs/blob/master/l
 │   ├── RESOURCES.md
 │   └── TESTS.md
 └── exercises
-    ├── accumulate
-    │   ├── package.yaml
-    │   ├── stack.yaml
-    │   ├── src
-    │   │   └── Accumulate.hs
-    │   ├── test
-    │   │   └── Tests.hs
-    │   └── .meta
-    │       ├── examples
-    │       │   └── success-standard
-    │       │       ├── package.yaml
-    │       │       └── src
-    │       │           └── Accumulate.hs
-    │       └── hints.md
-    ├── allergies
-    │   └── ...
-    └── ...
+    └── practice
+        ├── accumulate
+        │   ├── package.yaml
+        │   ├── stack.yaml
+        │   ├── src
+        │   │   └── Accumulate.hs
+        │   ├── test
+        │   │   └── Tests.hs
+        │   └── .meta
+        │       ├── examples
+        │       │   └── success-standard
+        │       │       ├── package.yaml
+        │       │       └── src
+        │       │           └── Accumulate.hs
+        │       └── hints.md
+        ├── allergies
+        │   └── ...
+        └── ...
 ```
 - `config.json`: Every exercise has to be registered here. It has a unique name and a difficulty. The sequence order is also the default order in which the exercises are fetched.