Update license copyright line, wording, and file extension

[mbrukman]

The current version of LICENSE.md has a few issues:

1. the copyright is listed as "Copyright (c) 2013, Cyrille Rossant" but there have been other contributors since then besides Cyrille

   Solution options: (a) we can switch this to "Copyright (c) 2013 Cyrille Rossant and contributors" or (b) as some projects do this "Copyright (c) 2013 ipycache authors" with a relevant AUTHORS.txt and CONTRIBUTORS.txt in the repo to clarify. @rossant – I'm fine with (a) if you'd like to keep your name in the copyright line as the original author and primary contributor.

2. There's strange wording in the license:

   ipycache/LICENSE.md

   Lines 16 to 18 in 4042ed1

   	* Neither the name of border nor the names of its contributors
   	may be used to endorse or promote products derived from this
   	software without specific prior written permission.

   This looks like the license text was copied from another project or company where the name "border" was relevant, but the actual BSD-3-Clause license from OSI says:

   Neither the name of the copyright holder nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

   Solution: I would like to change this to match the original wording since this is not relevant.

3. the file extension is .md but it's really a text file and written as such, so it should be switched to a plain-text file

   Solution is easy: switch the file to .txt extension

@rossant — what are your thoughts on (1)? Would you prefer option (a) or (b)? I think items (2) and (3) are non-controversial; please let me know if there's some history I should be aware of for (2) and if you have any concerns about the proposed changes.

Thanks!

[rossant]

Looks good to me!
For the license, there might be a third option, something like this (IPython):

- Copyright (c) 2022-Present, ipycache development team
- Copyright (c) 2013-2022, Cyrille Rossant

what do you think?

[mbrukman]

@rossant — I looked at git history via git log --pretty="format:%an %ad", and found that the first commit by someone other than you was from 2013, not 2022, so I am going with your suggestion, but with s/2013/2022/, hope that's OK. Sending PR shortly.