Add better documentation about the templating system and the attribute parameters

[refnil]

@nmattia yes, it does (thank you by the way!), though I think the role of attributes should be clarified, at least on the readme, what do you think? The relationship between attributes and templates is never explained explicitly...

Originally posted by @sir4ur0n in #232 (comment)

[nmattia]

The --help says this:

 -t,--template URL        Used during 'update' when building URL. Occurrences
                           of <foo> are replaced with attribute 'foo'.

I guess something like this could be added to the README?

[vst]

And also what attributes are available by default, maybe?

I understand that name is not available in the template context, and must be provided additionally as --attributes name=....

Examples

For example, this does not work:

$ niv add \
  autodocodec --version 0.4.1.0 \
  --template "https://hackage.haskell.org/package/<name>-<version>/<name>-<version>.tar.gz"

Adding package autodocodec
  FATAL: One or more packages failed to update:
  autodocodec: Update failed: Could not render template https://hackage.haskell.org/package/<name>-<version>/<name>-<version>.tar.gz
  with keys: url_template, version

  CallStack (from HasCallStack):
    error, called at src/Niv/Update.hs:80:25 in niv-0.2.22-IlyoaYrYPiZATCJYABWS2Z:Niv.Update
ERROR: ExitFailure 1

This does not work either:

$ niv add \
  autodocodec --name autodocodec --version 0.4.1.0 \
  --template "https://hackage.haskell.org/package/<name>-<version>/<name>-<version>.tar.gz"

Adding package autodocodec
  FATAL: One or more packages failed to update:
  autodocodec: Update failed: Could not render template https://hackage.haskell.org/package/<name>-<version>/<name>-<version>.tar.gz
  with keys: url_template, version

  CallStack (from HasCallStack):
    error, called at src/Niv/Update.hs:80:25 in niv-0.2.22-IlyoaYrYPiZATCJYABWS2Z:Niv.Update
ERROR: ExitFailure 1

This does:

$ niv add \
  autodocodec --attribute name=autodocodec --version 0.4.1.0 \
  --template "https://hackage.haskell.org/package/<name>-<version>/<name>-<version>.tar.gz"

Adding package autodocodec
  Writing new sources file
Done: Adding package autodocodec

Let me know if you want me to issue a PR along this line.