docs: add trigger() function documentation to README

[Copilot]

Added documentation for the trigger() function that was introduced in the PR but not documented in the README.

Changes

• Added "Manual Triggering" section to README after "Effect Scope"
• Documented basic usage: manually triggering updates after direct mutations
• Documented multi-signal triggering pattern

Example

import { signal, computed, trigger } from 'alien-signals';

const arr = signal<number[]>([]);
const length = computed(() => arr().length);

// Direct mutation doesn't automatically trigger updates
arr().push(1);
console.log(length()); // Still 0

// Manually trigger updates
trigger(arr);
console.log(length()); // 1

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