Update documentation for compiling wasm library

[maxammann]

I tried to compile the wasm library myself and failed to compile it with fastcomp.

[vixentael]

Thank you @maxammann

Luckily, @ilammy has created cossacklabs/themis#780 that will allow us to test using 2.0.13 :)

[ilammy]

Indeed, support for “fastcomp” has been dropped with cossacklabs/themis#760. Though this change has not been made public in an official release, but builds from master branch pretty much require an “upstream” flavor now. Given that the docs describe building the latest version from source... Thanks for noticing this, @maxammann!

[maxammann]

Thank you @maxammann

Luckily, @ilammy has created cossacklabs/themis#780 that will allow us to test using 2.0.13 :)

I saw that PR and already bumped the docs to 2.0.13. Like a proper timemachine!