Fix typos in README.md

[melongbob]

• "a online" -> "an online"
• "a name and room number" -> "a name and a room number"
• "have no specific host of a room" -> "a room without a host" For the verb "have" it is better to "have something" than "have no something"
• "if a client joins a room late, I want them to" -> "if clients join a room later, they would want..." Subjects in the sentence was synchronized with "people" from the previous sentence for higher cohesion. Fixes #92