update CONTRIBUTING.md

Added basic version of contributor's checklist of things to pay
attention for when submitting a pull request.

CONTRIBUTING.md

@@ -155,6 +155,118 @@ Do not send disruptive changes, like without reason reverting commits or
 deleting files, creating spam content, etc. Authors of such pull requests may
 be blocked from contributing to [Termux](https://github.com/termux) project.
 
+### Submitting new packages: checklist
+
+Besides [packaging policy](#packaging-policy), there is a number of typical
+mistakes that could be made when submittung a pull request with new package.
+Pay attention to things listed below.
+
+1. **Versioning: format**
+
+   Package versions must begin with a number and should not contain special
+   characters except `.` (dot), `-` (minus), `+` (plus). Under certain cases
+   the colon symbol (`:`) is allowed - for specifying epoch.
+
+   Examples of valid version specification: `1.0`, `20201001`, `10a`.
+
+2. **Versioning: if using specific Git commit**
+
+   `TERMUX_PKG_VERSION` must contain a commit date in case if you are using
+   specific Git commit. Date format should be `YYYY.MM.DD` or `YYYYMMDD`.
+
+   Never use Git hash, branch name or something else that can break version
+   tracking in package manager!
+
+3. **Source URL**
+
+   Source URL must be deterministic and guarantee that it always pointing
+   on content matching version specified in `TERMUX_PKG_VERSION` and
+   checksum in `TERMUX_PKG_SHA256`. In very rare cases we could make
+   exception, but don't expect that it will apply to your pull request.
+
+   Don't hardcode version in source code URL. Reference it through variable
+   `${TERMUX_PKG_VERSION}` and remember that Bash supports slicing and
+   other ways to manipulate content referenced through variables.
+
+4. **Dependencies: build tools**
+
+   Don't specify common build tools in package dependencies. This includes
+   packages like `autoconf`, `automake`, `bison`, `clang`, `ndk-sysroot`
+   and many others.
+
+5. **Dependencies: build & run time**
+
+   `TERMUX_PKG_DEPENDS` should contain only dependencies required during
+   package run time.
+
+   All dependencies that are used only during build time, for example
+   static libraries, should be specified in `TERMUX_PKG_BUILD_DEPENDS`.
+
+6. **Patches: format**
+
+   Patches are standard diff output generated by GNU diff or Git. Please
+   avoid editing patches by hand, especially if you don't understand
+   format internals.
+
+   Patch is typically created by
+   ```
+   diff -uNr sourcedir sourcedir.mod > filename.patch
+   ```
+
+7. **Patches: hardcoded path references**
+
+   Software often relies on paths defined by Filesystem Hierarchy Standard:
+
+   * `/bin`
+   * `/etc`
+   * `/home`
+   * `/run`
+   * `/sbin`
+   * `/tmp`
+   * `/usr`
+   * `/var`
+
+   These paths do not exist in Termux and have been replaced by prefixed
+   equivalents. Termux installation prefix is
+   ```
+   /data/data/com.termux/files/usr
+   ```
+   and can be considered as virtual rootfs.
+
+   Home directory is stored outside of prefix:
+   ```
+   /data/data/com.termux/files/home
+   ```
+
+   Don't hardcode home and prefix, use shortcuts `@TERMUX_HOME@` and
+   `@TERMUX_PREFIX@` respectively. Patch files are preprocessed before
+   being applied.
+
+   Directories `/run` and `/sbin` should be replaced by
+   `@TERMUX_PREFIX@/var/run` and `@TERMUX_PREFIX@/bin` respectively.
+
+8. **Build configuration: compiler flags**
+
+   You should not touch `CFLAGS`, `CXXFLAGS`, `CPPFLAGS` or `LDFLAGS`
+   variables unless this is necessary to make build working.
+
+9. **Build configuration: autotools**
+
+   The `build-package.sh` does pretty much work to properly configure
+   package builds using GNU Autotools. Therefore you do not need to
+   specify flags like
+
+   * `--prefix`
+   * `--host`
+   * `--build`
+   * `--disable-nls`
+   * `--disable-rpath`
+
+   and some others.
+
+   Additional options to `./configure` can be passed through variable
+   `TERMUX_PKG_EXTRA_CONFIGURE_ARGS`.
+
 ***
 
 # Working with packages