Document installation instructions

[armfazh]

git pull origin master

npm run clean
npm run build

• if you have perl and make in PATH,
  then "sjcl" should build without any errors
• ./dist directory should exist and contain an unpacked webextension,
  which you can install in your web browser for testing,
  or use your web browser to pack into a self-signed extension

Originally posted by @warren-bank in #300 (comment)

[warren-bank]

quick note for context: in the issue this comment was taken from, the person i was replying to already had a local git repo cloned from github (as origin) that was behind by (at least) one commit.. which they needed to pull to fix the issue at hand; any documentation would (of course) start from scratch in a clean directory.

[warren-bank]

PS: somewhat (indirectly) related..

PR #283 has made some (minor) changes to the npm scripts:

• "prebuild" calls "clean".. so the user doesn't have to
• "clean" removes additional directories:
  • /lib that is generated by the typescript compiler
  • ..the others only apply to updates made by the PR and don't pertain to master (yet)
    • still waiting for movement on this PR..