Document requirement of Nightly compiler

[davelab6]

% git clone https://github.com/MFEK/stroke;
% cd stroke;
% cargo run --;
    Updating git repository `https://github.com/MFEK/math.rlib`
    Updating git repository `https://github.com/MFEK/glifparser.rlib`
    Updating git repository `https://github.com/MFEK/flo_curves`
  Downloaded xmlwriter v0.1.0
  Downloaded chunked_transfer v1.4.0
  Downloaded unicode-width v0.1.8
  Downloaded unicode-normalization v0.1.17
  Downloaded aho-corasick v0.7.15
  Downloaded clap v2.33.3
  Downloaded which v3.1.1
  Downloaded unicode-bidi v0.3.4
  Downloaded humantime v2.1.0
  Downloaded webpki v0.21.4
  Downloaded xmltree v0.10.2
  Downloaded xattr v0.2.2
  Downloaded tinyvec v1.1.1
  Downloaded textwrap v0.11.0
  Downloaded tinyvec_macros v0.1.0
  Downloaded percent-encoding v2.1.0
  Downloaded lazycell v1.3.0
  Downloaded glob v0.3.0
  Downloaded vec_map v0.8.2
  Downloaded termcolor v1.1.2
  Downloaded form_urlencoded v1.0.1
  Downloaded url v2.2.1
  Downloaded shlex v0.1.1
  Downloaded tar v0.4.33
  Downloaded strsim v0.8.0
  Downloaded webpki-roots v0.21.0
  Downloaded clang-sys v1.0.1
  Downloaded untrusted v0.7.1
  Downloaded libloading v0.6.7
  Downloaded peeking_take_while v0.1.2
  Downloaded env_logger v0.8.3
  Downloaded ansi_term v0.11.0
  Downloaded atty v0.2.14
  Downloaded rustls v0.19.0
  Downloaded bindgen v0.57.0
  Downloaded skia-safe v0.38.3
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  Downloaded ring v0.16.20
  Downloaded roots v0.0.6
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  Downloaded ureq v2.0.2
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  Downloaded heck v0.3.2
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  Downloaded 48 crates (7.8 MB) in 1.28s (largest was `ring` at 5.1 MB)
   Compiling cfg-if v1.0.0
   Compiling libc v0.2.91
   Compiling memchr v2.3.4
   Compiling cc v1.0.67
   Compiling log v0.4.14
   Compiling spin v0.5.2
   Compiling untrusted v0.7.1
   Compiling glob v0.3.0
   Compiling matches v0.1.8
   Compiling version_check v0.9.3
   Compiling tinyvec_macros v0.1.0
   Compiling bitflags v1.2.1
   Compiling proc-macro2 v1.0.24
   Compiling autocfg v1.0.1
   Compiling serde v1.0.125
   Compiling unicode-width v0.1.8
   Compiling regex-syntax v0.6.23
   Compiling crc32fast v1.2.1
   Compiling ryu v1.0.5
   Compiling percent-encoding v2.1.0
   Compiling unicode-xid v0.2.1
   Compiling ansi_term v0.11.0
   Compiling base64 v0.13.0
   Compiling serde_json v1.0.64
   Compiling bindgen v0.57.0
   Compiling strsim v0.8.0
   Compiling termcolor v1.1.2
   Compiling adler v1.0.2
   Compiling humantime v2.1.0
   Compiling vec_map v0.8.2
   Compiling lazycell v1.3.0
   Compiling rustc-hash v1.1.0
   Compiling itoa v0.4.7
   Compiling lazy_static v1.4.0
   Compiling chunked_transfer v1.4.0
   Compiling shlex v0.1.1
   Compiling peeking_take_while v0.1.2
   Compiling unicode-segmentation v1.7.1
   Compiling once_cell v1.7.2
   Compiling either v1.6.1
   Compiling xml-rs v0.8.3
   Compiling roots v0.0.6
   Compiling smallvec v1.6.1
   Compiling xmlwriter v0.1.0
   Compiling unicode-bidi v0.3.4
   Compiling libloading v0.6.7
   Compiling tinyvec v1.1.1
   Compiling textwrap v0.11.0
   Compiling form_urlencoded v1.0.1
   Compiling nom v5.1.2
   Compiling itertools v0.9.0
   Compiling miniz_oxide v0.4.4
   Compiling clang-sys v1.0.1
   Compiling heck v0.3.2
   Compiling xmltree v0.10.2
   Compiling unicode-normalization v0.1.17
   Compiling ring v0.16.20
   Compiling glifparser v0.0.0 (https://github.com/MFEK/glifparser.rlib#ee7ddff2)
   Compiling aho-corasick v0.7.15
   Compiling atty v0.2.14
   Compiling which v3.1.1
   Compiling filetime v0.2.14
   Compiling xattr v0.2.2
error[E0554]: `#![feature]` may not be used on the stable release channel
 --> /Users/dcrossland/.cargo/git/checkouts/glifparser.rlib-8a794fa86f0b184f/ee7ddff/src/lib.rs:1:1
  |
1 | #![feature(assoc_char_funcs, let_chains)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

   Compiling quote v1.0.9
   Compiling clap v2.33.3
error: aborting due to previous error

For more information about this error, try `rustc --explain E0554`.
error: could not compile `glifparser`

To learn more, run the command again with --verbose.
warning: build failed, waiting for other jobs to finish...
error: build failed
%

[ctrlcctrlv]

The build failure here has the same cause as MFEK/glif#97, but the requirement of the Nightly compiler is here undocumented and needs to be.

assoc_char_funcs would be harder for us to do without, but let_chains might be fine to remove.