Add missing from_source manual page

manual/src/from_source.md

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+# Installing From Source
+
+### Build Requirements
+
+Difftastic is written in Rust, so you will need Rust installed. I
+recommend [rustup](https://rustup.rs/) to install Rust. Difftastic
+requires Rust version 1.65 or later.
+
+You will also need a C++ compiler that supports C++14. If you're using
+GCC, you need at least version 8.
+
+### Build
+
+You can download and build [difftastic on
+crates.io](https://crates.io/crates/difftastic) with Cargo (which is
+part of Rust).
+
+```
+$ cargo install --locked difftastic
+```
+
+Difftastic uses the `cc` crate for building C/C++ dependencies. This
+allows you to use environment variables `CC` and `CXX` to control the
+compiler used (see the [`cc`
+docs](https://github.com/alexcrichton/cc-rs#external-configuration-via-environment-variables)).
+
+See [contributing](./contributing.md) for instructions on debug
+builds.
+
+## (Optional) Install a MIME Database
+
+If a MIME database is available, difftastic will use it to detect
+binary files more accurately. This is the same database used by the
+`file` command, so you probably already have it.
+
+The MIME database path is [specified in the XDG
+specification](https://specifications.freedesktop.org/shared-mime-info-spec/0.11/ar01s03.html). The
+database should be at one of the following paths:
+
+* `/usr/share/mime/magic`
+* `/usr/local/share/mime/magic`
+* `$HOME/.local/share/mime/magic`