Add warning text about curl install method

Installing software by piping random scripts into a shell is a dangerous
way to install software. Although it is convenient to have a simple
install procedure like that, it is highly discouraged in most tech
circles and I would not like to perpetuate the cycle. Adding the warning
disclaimer allows me to continue offering the curl install procedure
while educating users that it is an unsafe way to install software and
lets me point them towards safe install procedures.
Clarify fishmarks update instructions

The update instructions did not account for users whose fishmarks
install locations were not the default ~/.fishmarks, but the new
instructions note that ~/.fishmarks is just the default and include code
demonstrating how to change into the install directory. The instructions
for step 2 and 3 explain the function of each of the Git commands.
Add Contributing and Todo section to README

Add guidelines on filing issues, submtting feature requests, fixing
issues, adding features, and writing good commit messages to foster
development and contribution. Also add a Todo sections outlining future
plans for fishmarks.