update doc

src/beam.py

@@ -71,7 +71,17 @@ def main():
 
   parser_a = subparsers.add_parser(
     'frequency', help="eigen frequencies", prefix_chars='-',
-    description="Eigen frequenccies of an Euler-Bernoulli beam.")
+    description=r"""Eigen frequencies of an Euler-Bernoulli beam.
+
+     βₙ² ⎛E I⎞¹/₂           π
+fₙ = --- ⎜---⎟    with βₙ = -- aₙ
+     2π  ⎝ρ A⎠              L
+
+aₙ is a numerically solved value.
+
+!!! Currently only the first four frequencies can be calculated.
+""",
+    formatter_class=argparse.RawTextHelpFormatter)
   parser_a.add_argument('N', type=int, help='Mode number')
   parser_a.add_argument('E', type=float, help='Young\'s modulus')
   parser_a.add_argument('I', type=float, help='Second moment of area')