readme: document enchant dependency

Author: oliver-sanders

README.md

@@ -84,7 +84,20 @@ We use a few custom Sphinx extensions, for details see
 
 ### Installation
 
-Note: you may need to install `graphviz` first (e.g. `conda install graphviz`).
+### Non Python Dependencies
+
+There are two non-Python dependencies which must be installed:
+
+* `graphviz`
+* `enchant`
+
+E.G. with Conda/Mamba:
+
+```console
+$ mamba install -c conda-forge graphviz enchant
+```
+
+### Checkout & Install
 
 ```console
 $ git clone [email protected]:cylc/cylc-doc.git cylc-doc

src/tutorial/scheduling/datetime-cycling.rst

@@ -156,11 +156,8 @@ Recurrence Formats
 
 ``2000/P4Y``
    Every fourth year, starting with the year 2000.
-``2000-01-01T00/T00``
+``2000-01-01T00/P1D``
    Every day at midnight, starting on the 1st of January 2000.
-``2000-01-01T12/T00``
-   Every day at midnight, starting on the first midnight
-   after the 1st of January at 12:00 (i.e. ``2000-01-02T00``).
 
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