Remove nanosecond datetime tests, document implementation-defined beh…

…aviour

I considered copying this to a special "optional" directory or
something, but it gets very complicated very fast with different
permutations. There's no telling what precision an implementation
supports.

So just document it. It makes things easier for everyone.

Fixes #91

README.md

@@ -195,6 +195,19 @@ An example implementation can be found in the BurnSushi/toml:
 - [Add tags](https://github.com/BurntSushi/toml/blob/master/internal/tag/add.go)
 - [Remove tags](https://github.com/BurntSushi/toml/blob/master/internal/tag/rm.go)
 
+Implementation-defined behaviour
+--------------------------------
+This only tests behaviour that's should be true for every encoder implementing
+TOML; a few things are left up to implementations, and are not tested here.
+
+- Millisecond precision (4 digits) is required for datetimes and times, and
+  further precision is implementation-specific, and any greater precision than
+  is supported must be truncated (not rounded).
+
+  This tests only millisecond precision, and not any further precision or the
+  truncation of it.
+
+
 Assumptions of Truth
 --------------------
 The following are taken as ground truths by `toml-test`:

tests/valid/datetime/milliseconds.json

@@ -1,18 +1,18 @@
 {
   "utc1": {
     "type": "datetime",
-    "value": "1987-07-05T17:45:56.123456Z"
+    "value": "1987-07-05T17:45:56.1234Z"
   },
   "utc2": {
     "type": "datetime",
-    "value": "1987-07-05T17:45:56.600000Z"
+    "value": "1987-07-05T17:45:56.6000Z"
   },
   "wita1": {
     "type": "datetime",
-    "value": "1987-07-05T17:45:56.123456+08:00"
+    "value": "1987-07-05T17:45:56.1234+08:00"
   },
   "wita2": {
     "type": "datetime",
-    "value": "1987-07-05T17:45:56.600000+08:00"
+    "value": "1987-07-05T17:45:56.6000+08:00"
   }
 }

tests/valid/datetime/milliseconds.toml

@@ -1,4 +1,4 @@
-utc1  = 1987-07-05T17:45:56.123456Z
+utc1  = 1987-07-05T17:45:56.1234Z
 utc2  = 1987-07-05T17:45:56.6Z
-wita1 = 1987-07-05T17:45:56.123456+08:00
+wita1 = 1987-07-05T17:45:56.1234+08:00
 wita2 = 1987-07-05T17:45:56.6+08:00