docs(readme): add day of month and day of week OR condition behavior

Clarifies that when both fields are specified with specific values,
they create an OR condition (not AND), following standard POSIX cron behavior.

Also mentions the # character syntax being considered in #877 as an
alternative for patterns like "first Monday of the month".

Closes #890

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: hackertaco

README.md

@@ -280,6 +280,16 @@ day of week    0-7 (0 or 7 is Sunday, or use names)
 
 - Using arrow functions for `onTick` binds them to the parent's `this` context. As a result, they won't have access to the cronjob's `this` context. You can read a little more in issue [#47 (comment)](https://github.com/kelektiv/node-cron/issues/47#issuecomment-459762775).
 
+- When both `day of month` and `day of week` are specified with specific values (not `*`), they create an **OR condition**, not AND. This follows the standard POSIX cron behavior. For example:
+  - `0 0 1-7 * 1` means "run at midnight on days 1-7 **OR** on any Monday", not "run on Mondays that fall within days 1-7".
+  - `0 0 15 * 5` means "run on the 15th of any month **OR** on any Friday".
+
+  If either field is set to `*` (unrestricted), the relationship becomes AND:
+  - `0 0 15 * *` means "run on the 15th of any month" (day of week is unrestricted).
+  - `0 0 * * 1` means "run on any Monday" (day of month is unrestricted).
+
+  **Note:** Patterns like "first Monday of the month" cannot be directly expressed with standard cron syntax. A `#` character syntax (e.g., `0 0 * * 1#1` for "first Monday") is being considered in [#877](https://github.com/kelektiv/node-cron/pull/877). See [#890](https://github.com/kelektiv/node-cron/issues/890) for more details.
+
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