Add "no file descriptors available" description and solution to README.md

[epi052]

Depending on a user's operating system's open file limit, the user may see a bunch of no file descriptors available errors. A user can increase the limit by editing limits.conf or using ulimit.

I'd like a description of the problem and both solutions to be included in the readme.

Definition of Done:

• entry in table of contents, something like Common Problems or similar
• sub-entry in table of contents to the above that indicates this particular problem (no file descriptors available)
• H2 heading that corresponds to the Common Problems or similar entry (include relevant emoji, warning symbol or something?)
• H3 heading that corresponds to the particular problem
• Under the H3 heading, a description of the problem and both limits.conf and ulimit solutions

Relevant discussions:

• https://github.com/epi052/feroxbuster/issues/77#issuecomment-710790100
• https://forum.hackthebox.eu/discussion/comment/85185

[epi052]

Closed by 404b231c6732d89a1d35c39d89704d0b8be5204f