fix: make sure folks know that tehy have complete control over their username

src/register.html

@@ -18,6 +18,10 @@
           Thank you for signing up for your Skynet account.
           <br/>
           Please complete the process by filling in the form below.
+          <br />
+          Username is what you want to be known on Skynet as.
+          <br>
+          It must be lowercase, start with a letter and contain only 0-9 a-z and _-
         </p>
         <form id="register">
           <table id="table">