What is the red-headed league? How does Jabez Wilson find out about it? What are the conditions for getting and keeping a place in the league?

Jabez Wilson finds out about the Red-Headed League through an advertisement which had been placed in the newspaper. This advertisement said that a salary of £4 per week would be payable to members of the League who met its conditions, which were:
1. Members had to be sound in body and mind;
2. Members had to be over the age of twenty-one years, and;
3. Members had to be red-haired.
The advertisement also stipulated that membership of the league was open only to men, suggesting that red-haired women would not be eligible.
£4 a week was a significant sum in those days, which is why the advertisement caught Wilson's attention. Those wishing to become a member of the League would have to perform "nominal services" and would have to make an application in person at a stated time and place.
The League, supposedly, was founded by a red-haired American gentleman who wanted to leave his fortune to other men with red hair. When Wilson applies and is accepted, he is told that the work entails copying out the Encyclopaedia Britannica four hours a day.
This, naturally, struck Wilson as strange, particularly when, one morning, he arrived and found the offices closed and locked. In the course of his investigation, Holmes discovers that the League is in fact an invention; the criminals who thought of it had simply wanted to keep Wilson out of his shop so that they could receive stolen goods from a bank vault through a tunnel in the floor of said shop.