Added CYLC_SOURCE_DIRECTORY template variable to jinja2 doc (#868)

* Added CYLC_SOURCE_DIRECTORY template variable to jinja2 doc

* Update src/user-guide/writing-workflows/jinja2.rst

Co-authored-by: Oliver Sanders <oliver.sanders@metoffice.gov.uk>

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Co-authored-by: Oliver Sanders <oliver.sanders@metoffice.gov.uk>

Author: wxtim

src/user-guide/writing-workflows/jinja2.rst

@@ -139,6 +139,7 @@ Jinja2 CYLC variables available when parsing any workflow (source or installed):
    ======================    ==============
    CYLC_VERSION              Version of Cylc parsing the configuration
    CYLC_WORKFLOW_NAME        Workflow name (source, or run ID minus run name)
+   CYLC_WORKFLOW_SRC_DIR     Path of the source directory.
    CYLC_TEMPLATE_VARS        Variables set by '--set' CLI options or plugins
    ======================    ==============